In general I like your idea, mostly because I´ve had similar issues and posted ideas about revolts in EU4 ;-) I will address the issues by what I believe could be the trouble and the solution to it. But in general, a revolt system that allows for single planets to break free at times without making it impossible to hold together an empire due to single revolts getting independent each time. And the chance for a massive uprising with different factions, some even helped by outsiders, that might share a common goal or have differing goals.
First off, revolting in general:
Information is key, but as you I dislike too much information and too little randomness. In EU4 you can see how close a province is to revolting, how many men you need to make it decrease or how long you can wait for it to blow off by itself. However we need to have the knowledge of an area being conflicted, but this could be done with more fluid solutions that makes players guessing and fearing when and where.
A
two-part mood system could solve this, by having the first mood show if they are friendly, indifferent or hostile towards you which would give you some idea if this area is or could be troublesome. The second would be something like going from "People doesn´t care", "People are upset" to "people are mad" at which revolt should and could happen by chance in either with an increase in chance to reflect a massive empires incapabilities to know every detail of every group. So a planet that seems friendly and happy could have a revolt to which they could grow independent, where the benefits afterwards could be, that you still have loyalist among the POP and can use this as a diplomatic benefits or they might rise up in order to return.
Planetary revolting:
Securing a massive planetary empire is expensive and troublesome, since you can´t pull every unit into every war without any risk of uprisings due to the lack of imperial presence. Something the EU system lacks, since you can go to war just after revolts or you know they are too far off to manage to do anything about it. Yet every nation should have the fear of revolts rising if not super stable or simply due to need of going to war taking the chances that revolts will not happen.
So either you can´t put your whole army into the war in fear of a few system revolting, which makes the need for a fleet nearby to quell them swiftly, or you risk it since the war is taking it´s toll and you need that extra ships to tip the balance in your favour. Strategic choices I would love, though I would hate when I misjudge and revolts starts popping around I´d learn to manage since it was my choice to pull the fleet.
To this end, imperial presence or the absence to precise, should have an effect as should the Imperial capabilities to fight the rebellion, but not restricted to it. Since by ideology they could be so sick of the government or their ethos that they take their chances anyway hoping for outside help if they haven´t got the promise already for support. So the likely scenario of planetary independence should be when imperial presence is low and they are able to take over control from the few remaining soldiers and police forces present by default. This leads to my thoughts on win condition which comes below.
Independence should be much more fluid and aquired somewhat earlier, but not necessarily mean peace between the parties. In real-life they would establish a government soon after taking the planet, yet it would take a while to gain the "full" control and establish a functional society required to be independent. So instead of waiting 5 years (EU4) where you can annull the period by sieging back the "province" while the rebels are stil alive, you would have to not only occupy the province but defeat the rebels. So having a single unit following the rebels behind just to reset the countdown until your forces can get there shouldn´t be enough. Also do the improved rebel AI that should recoignize that a single unit is no match for a 15 unit rebel army and simply wipe the "follower" out.
Government and it´s diplomacy:
Independence should be granted faster to allow more diplomatic options with it and stop the exploit from EU4 where a 1 unit follower can reset the timer. It should take a while to establish a government, but having a goverment is not the same as having a functional society. Recent history has shown us how fast rebels can form a government even with minimal controlled areas, what comes next is trying to make it functional like healthcare, social services, taxes and payments. But before this, a government would be needed and would be reflected as the first step towards total independence and be grounds for a more varied diplomatic interactions against the empire and it´s enemies.
Diplomacy can start before the revolution and will be deepened below (yes, sry for the long post) but the gist of it is of course "support rebels" to which a few actions should be available along with inciting revolt and sending troops in to support and fire the revolt. This would of course have an effect on the relation when the government is formed, since they already have had dealings and their support would still be welcome and perhaps much needed. However the rebel-based diplomacy should be somewhat limited, though it could be cool to get a notice from an upcoming rebelion that asks for support, this is unlikely and would most likely be the neighbouring empire who reach out to the rebels instead seeing how they have a common interest.
After initial rebelion, like 4-6 months after the rebels would form a government and then perhaps try to conquer other planets nearby, or it´s a system-wide uprising with multiple revolts taking control of several planets at once, they would form into a united government adding to the first planet that established it. Most importantly, this would mean that the rebels would be able to make diplomatic approaches to other empires, instead of waiting for them to ask to support them, they could instead ask other to support their cause which could lead to alliances if revolution is succesful, but not always depending on their relations to other nations too. Since the common goal is when the war is over and the former "ruler" is no longer there to support against, thus making "normal" diplomacy come into effect, meaning that allying their rival would mean they would dislike the government though having helped against their other rival.
Granting a government and semi-independence grants some other benefits, like restricting the follower tactics. Since the rebels now functions under nation AI and react like them, they would destroy the following unit, also it would mean that the follower would only occupy and not regain the planet. In effect, the rebels have gained an independent government which are at war with their former "ruler", this means peace must be made before the revolution is over instead of simply destroying the rebels and regaining the lands. As stated, you only occupy their provinces not regain full control of them, making them work just like a war with another nation.
This means that the rebels also will try to take over other planets to which those of similiar ethos or ethnicities with a low desire to stay in the empire will have a high risk at flipping shortly after occupation, perhaps with a cool-down to construction of units, and those planets with a high desire to stay in the empire or at least have a lower desire to join the rebellion would be occupied as a normal war, being subject to the upcoming peace-deal. So a system or string of systems with similar background could swiftly grow from a single revolt, into a fearsome entity that´s not to be trifled with.
This also means that from forming the government, they will have access to building new and more powerful units that they couldn´t when being a secret faction. A rebellion would have a hard time hiding that star destroyer in their basement, so it makes little sense to have them pop up magically, unless of course the empire has been extremely neglect or just that dismissive. However it doesn´t make sense either, that the rebels that have controlled an area for a while doesn´t start to use their resources to better their chances, to which starting the construction of a star destroyer could mean they´d stand a chance at expanding. This however should start out slow, mainly due to society not being fully functional and the need for more important war materials could mean that the star destroyer would wait a bit.
Evolving revolts and massive uprisings:
Mentioned above, a single revolt could grow massively if gone unchecked, especially in areas where other empires have been subjugated since the risk of joining the revolution is increased. However another mechanism along with massive uprisings should be implemented,
increased risk upon revolt and it´s succes. If a revolt is starting to gain ground without being countered, other rebels would see this as a sign of a weak empire and then more likely start rebelling on their own, also rebels in other systems and planets that share their ethnicity or ethos would more likely rebel to in order to either join or gain their own independence. This would also mean that, but not restricted by, the planets and systems closest to the revolt and furthest from the imperial core and/or armies would have an increased chance of revolting.
This could be where the rebel diplomacy could be great, since those rebels that share ethos but not willing to band together under one banner would be more willing to ally against their common foe in order to further their own goals, instead of being to single entities that work individually and thus are easier to put down. This would be solved by the rebel sub-category below, which could have a series of outcome depending on the circumstances and their willingness to band together.
So revolts can evolve into massive uprising, which should be difficult to put down and further cement the need for player and AI to fear and react to rebels and the threat of these. However, massive uprisings could and should occur on it´s own. This would mostly be due to a faction growing too powerful binding them together with a common goal or the simple fact that the POPs are tired of the government and it´s actions, like going to war all the time, perhaps it might be due to the simple fact that the empire is weak due to an ongoing or recently ended war that went bad making the rebels either seeing a chance or simply deeming the emperor or government weak and should be replaced.
Rebel category and sub-category:
This is very important for the rebel interactions and what outcomes should be from rebels succeding and aiding each other. So by dividing them into categories and sub-categories, the outcomes can become more clearly defined and whether their goals are similar, conflicting or benefitional. First off, I always thought it odd that religious rebels
always deemed it necessary to convert
all of the empire with the result of having a counter-revolution converting it back. While at times this makes sense and it could be logical goal for them, they should be able to settle for less like gaining an independent state.
While the "religious" rebels having over 50 % POP followers, their goal should be to change the government in general. But if the war drags out, or they can´t gain enough ground to reach this goal they might settle for splitting the empire giving their followers land to settle in, giving them rights or small changes to government, the absolutes are great initially but logically these will diminish if their ultimate goal cannot be achieved or they can gain it by other means.
That means, that every faction should have the ability to simply split off due to prolonged conflict where the empire has not been able to retake his position and simply end in a status quo where either side likes the deal but must, for the time being, respect the situation as it is. By effect, the land gained and owned (due to the new rebel-flip mechanic) would be kept and the conflict, due to special CB for both could erupt at a later point until they either decide to give it up and respect each other or simply needs to do so because of a mutual threat.
The other and more common rebel category is the individual rebels with differing goals that might be similar or in conflict with each other, along with their ethos and ethnicity they can be friendly or hostile to each other which might even be based on circumstances. Like to races not liking each other, with different governments and likings could still be "friendly" and helpful due to their territorial goals are not conflicting and due the need of cooperation since fighting individually against an empire could mean loss for both, but for this special occassion, they could both benefit from working together against the empire in order for both of them to gain their independence.
This however, could also end in the two factions duking it out due to too many or too far conflicting interests, goals or beliefs. Since one of them will have no dealings with such a filthy life-forms that follows strange, mythical beings, or they simply follows two different beliefts which cannot cooexist. Like two factions that want a governement change can´t work together to change the government, not even if the war drags out their secondary goal would be to at least break free individually.
So this would only be when the rebels falls under the same category and their sub-category then defines how likely they are to help each other along with their conflicting goals or shared interest. It makes little sense in reality, that two independent factions that doesn´t share territorial claims and doesn´t have beliefs that flatout makes interactions impossible, doesn´t try to work together in order to gain their mutual independence. Just as it doesn´t make sense, that two factions that work together are forced to become one nation with a later revolt, yet at times this could be the outcome due to rebel compromise or a strong militaristic and aggressive neighbour that would like to devour them singledly.
Diplomacy, peace and compromise:
First off, diplomacy starts at government creation where they also work on getting society to function (some modifiers that burns off gradually). This means they can ask for alliances or support from other nations (if a modifier like EU4 prevents alliances during war) but most importantly it makes diplomatic negotiations with the former "ruler" an option. This means, that while it´s still a rebellion they change from being colourless generics, into a nation at war with the former "ruler" with their own planets in control and peace can be made with various outcomes.
This solves the issue with the 1 unit follower, since it functions as a regular war where the empire must not only take the planets back, but be able to force his demands through by making the rebels accept his rule. In reality rebels aren´t over just because a battle has been fought, the rebel government must be forced to accept their surrender which must be could be done on the battlefield, but more likely by occupation. This also fixes the issues I have with rebel AI, since they would use the regular AI and try to occupy other planets not in their immidiate interests, both to stand a chance of making peace but perhaps in an ambitious attempt to expand.
This also means, that instead of a "white peace" leading to rebel surrendering, this would make the rebels still independent with both parties having a special CB for their previous claims. An empire that can enforce their demands, means that the rebels surrender their government and relinquish control but a rebel government that at least can hold the line against the empire or perhaps even defeat him can make different peace deals, like the compromise that differs from status quo in that no treaty is ever signed leaving the issue open for later dispute.
Compromises can come in various forms, like the faction gaining rights or changing rights into their likings, it could also means that they special system-rights are invoked like a system with racist xenos gain the rights to prohibit "strangers", it could be allowing the rebel government to remain as a puppet government like a vassal where they are still part of the empire but have a greater amount of autonomy. Since, in my opinion "vassals" should come in differing forms depending on autonomy this could mean that they gained the right to regulate themselves, have an army, make trade agreements or they might be allowed to as autonomous that they can declare war independently where the emperor could be called. This however, at the risk of them getting so powerful that a vassal-crisis happens, in which they stop paying tribute, make alliances with his rivals or simply declare war for full independency or simply to gain planets or systems that the emperor holds.
Compromises could also come in form of rebel interactions, since some rebels goal or interest could be to join the neighbour, at least as a vassal while some other would not likely do this just like they can make deals with other similar factions. This would allow for counter-deals to be made, like approaching the neighbour for his support by aggreing that they will become a vassal to them at the end of the war (this should mostly be when the neighbour openly engages in the war) or they can offer trade rights and post-alliances if they support them, perhaps even a few of those tech they have learned due being part of an advanced empire.
Rebel-to-rebel compromises could come in the terms of not retaking conflicting goals but work towards independence (and take the dispute later), they could agree to join their nations under one government in order to gain a better chance or simply co-existing governments like making alliances with each other. Like nations in general, such alliances and compromises can later turn sour, especially when they win against the emperor, they need to split the spoils or find out who really have the right to that planet in question.
Support rebels:
In EU4 and CK2 in particular, supporting rebels are extremely limited both in terms of how and what it does. This means, that support could come in forms of money, units, alliances or the similar types that works well. Before the revolt, this would have an increased chance of rising and the more support the more the supporter can make the rebels rise actively. Like giving them support of money and sending units for them to use if they revolt. This covert actions should of course have a risk of discovery and period to gain effect, since money can be transferred fairly easily shipping units and denying involvment can be bit more tricky. Yet, by supporting units (which is not direct support in the form of declaring war) the rebels must use these to gain their demands.
In effect, this would mean that even a smaller faction can become a threat if backed by another empire, since they will have an increased number of powerful units than they initially could rise, but the subject of hiding a star destroyer in the basement still stands, but could at least be solved by making it transported
after the rebellion, which would require some time to pass until it gets there, perhaps the rebels has already been given "control" of it in the supporters space, learning how to use it and manning it with their own crew while this would still be subject to transportation time. Another issue is that supporting troops should have some limits, since an empire hiding
all it´s units on a single planet or in a single system is not only unbalanced but illogical. While the empire could pay for the upkeep, some sort of force limit should apply since the rebels only have that many soldiers to man the ships and if the supporter keeps control of it it should be done by an open declaration of war/support.
Risk of discovery would involve the crisis system I´ve discussed somewhere else, since discovering your neighbour shipping troops, ships and money to a system should have some consequences. First of, the discovery should allow for the decision to send it back, destroy it, allow passage or capture it. The last should be done actively with a fleet and could make sense when the supporter denies involvement, yet have some tech you could scavenge from the ship. Since it´s either deemed an unlawful encroachment or simply pirates or rebels, the right of the denying supporter is not broken since they usually would have removed any signs of their involvement other than ship-design who could give it away. They may however be a bit negative (secretly) that you privateered their ship.
Upon discovery, it would also be more logical that a rebellion is under way. Thus you could choose to root it out or simply hope that you stopped it in time and simply position your fleet nearby, just in case. Rooting them, could have a negative effect though, since the population in general would feel it a violation and might just join the rebels though on the other hand, having a sizeable fleet needing to be stationed nearby for a revolt that might never happen (either due to stopping it, the imperial presence or simply luck) could be equally painful. Especially if you want to go to war or worse, have been declared war upon and needs the fleet elsewhere... Decisions, decisions
Crisis system:
A small summary according to these mechanics, since discovering a ship in your area your decision would mean alot depending on how the supporter reacts. The summary goes, that it might not be a supporter, it could simply be a hostile colonizer or simply a proxy war, meaning they take over a single system or planet which isn´t a war but you can react to i accordingly by sending fleets against them to protect your interests or demand them to withdraw or be seen as a declaration of war, to which allies come into play. This also means that patrols on the outskirts becomes more important, since loosing a few planets like this could be devastating or having them send ships in support of rebels could mean a revolt is coming shortly with increased power (thus limiting your fighting force since you cannot be everywhere at once).
In accordance to rebels and support, this means not only a crisis involving rebels, like policing them increases revolt risk and numbers or make the galactic community like you less since you treat your POP poorly. But also depending on how much you discover or how much the supporter reveals of their involvement, since knowing that your enemy is behind and taking his ship or destroying it could be a reason for war to both parties due to encroachment and damage to property to which reparations can be demanded (along with those planets you like

) these can of course be demanded in order to prevent a war, to which if supporter is discovered he will either not be able to support for a while or being discovered again could have more severe consequenses. But if the supporter is stronger, perhaps due to another on going war, the threat of having a two-front war could be too severe, meaning you actually need to pay the price of the ship along with not demanding they continue to support, meaning that it might have stopped now but they can resume it after if they like.
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Sorry for the long post, but liked to address the issue properly instead of just saying +1 ;-)