I like the idea of an imperial/bureaucratic government type where it would be appropriate, but I have some reservations towards some of the suggestions here and some of them would be very hard to implement in CK2.
Vassals:
- Family palaces, while a nice idea, are something that the devs have said would be a nightmare to untangle from the MR code, so their reusability here is unlikely to be possible. Even if they were possible to implement, the lack of trade post management and an election to try to win would probably make the game rather boring as your primary goal would be to get out of that situation. I also suspect that there would be a problem with the number of families slowly spiralling out of control over time, which would cause a slowdown and make it less likely that a particular family would end up landed (making you have to spend more time to sit around hoping the emperor lands you), unless there was something forcing the emperor to land a particular family, which could make little sense if he dislikes them, they have claims on him, or the like.
- Appointment would be great, though due to the (very probable) lack of family palaces you'd need to ensure that someone's last county can't be revoked without a good reason as and is inherited normally that would give them a game over if they are a player. However, this would possibly create issues if a vassal gives a title to a sub-vassal as it still should revert to the emperor but the "Don't take someone's last county" clause would mean that a vassal of the emperor would be able to grant each county to a separate vassal and have them keep it perpetually unless it is revoked or the family dies out. You would probably also need to disallow vassal warfare by default, at least where titles that were granted from the emperor is concerned, as you'd otherwise have vassals take land that another vassal temporarily was given because they fabricate a claim, inherited a claim somehow, or it is part of their de jure. There might also be issues arising from the fact that a human player would be very unlikely to invest in upgrading provinces that they won't inherit, which might make the empire have some barely developed provinces in its heartland even if it is doing very well.
- I'm not sure about doubling the demesne for vassals, as it would make it less likely that they'd need their own (non-baron-tier) vassals (which would give them less powerful vassals to deal with, which could be rather boring). While it would make vassals more powerful, which might make the empire more prone to civil wars (which might be good, if it happened in a manner that made sense), but it would also enable (and possibly encourage) granting vassals land very distant from the rest of their demesne (which realistically should be hard to manage) so that you could raise their levies elsewhere (which possibly could be prevented if there was a distance/adjacently limit on title grants).
- Bureaucratic vassals probably should not have their personal levies reduced by 40 %, as that would make them significantly weaker vs. the emperor, but giving them a smaller penalty (and smaller tax boost) and a penalty from their own bureaucratic vassals might be nice.
- Being able to ask for hereditary land would be nice (the current viceroyalty mechanic could use that, too), but it would probably need to be difficult to try and difficult to get it approved to prevent basically all appointed titles becoming feudal (though there probably should be something in place to prevent the emperor from always refusing such requests without a good reason to do so).
- Having cities as the preferred holding might be nice for vassals. They should probably also be allowed to hold castles (and temples, at least in the case of Muslim rulers), particularly as they'd otherwise suffer quite a bit if they became feudal and cities became worthless.
Emperor:
- Having to constantly grant counties could become annoying after a while, particularly as the empire grew in size. While this would give you an incentive to grant people titles permanently to cut down on the micromanagement, it would not necessarily be a good idea to have the feudalization of an empire depend on the player becoming bored (particularly as the AI wouldn't get bored), and unless council empowerment (at least when it comes to title grants) was forbidden you'd possibly end up with the emperor having a few counties too many at all times because the council doesn't let him grant them to anyone, partially because they like him less for having a too large demesne. You'd probably also need to rewrite quite a few events about granting land to vassals/the children of vassals to account for the fact that they shouldn't be granted in perpetuity, and deal with this when it comes to inheritance (as a landed bureaucratic heir probably shouldn't inherit his father's capital unless his father is feudal, but he should also not be allowed to make his bureaucratic land feudal upon inheritance without imperial permission).
- If the demesne size was doubled for the emperor as well, it would possibly make the empire more stable (which might be bad, as blob stabiliy already is an issue), but just doubling it for vassals while the emperor somehow is unable to manage more land would be rather strange.
- Reduced levies and increased taxes from bureaucratic vassals could be nice, as it would make blobs weaker and would make them have to rely more on mercs and retinues, but the AI would need to be able to handle it (e.g. having it realize that it has enough merc money that an opponent that is slightly stronger when counting current power should be relatively easy to defeat with mercs). It would also give you an incentive to sometimes grant requests to have titles turned into regular feudal titles to get more levies, even if this would have the obvious tradeoff of that vassal being able to grow stronger over time (which probably would be good, as it would make the choice meaningful).
- Allowing the emperor to hold cities as well as castles (and possibly temples, at least if Muslim) would be nice, but it might create some issues if they are the preferred holding if the empire was to turn feudal for one reason or another, particularly in the case of Constantinople as the Theodosian Walls either would start in a castle that isn't the capital holding (and thus wouldn't give as much protection for your family members if the capital is sieged down), which might end up granted to a baron by the player or the AI because they wanted to hold an extra county, or would start in a city that would stop being the capital (and thus would cause the same problem after the switch). This particular instance could obviously be handled by moving the Theodosian Walls to the proper holding when it becomes necessary, but more generally it might be problematic if you have to trade a city that is very developed for a castle that isn't, so perhaps cities should just be an allowed secondary holding instead of the preferred holding.
- While I think that some kind of decay/corruption/decline mechanic would make sense for empires (and possibly would help with the current issue where blobs generally tend to keep blobbing as they only grow stronger over time), I think that Decadence, as currently implemented, would be a terrible choice. Having a decay mechanic tied to the dynasty could create all kinds of problems if the family ended up ruling multiple empires (which is a problem for Muslims at the moment, as your Decadent family members in another realm somehow weaken your realm) and would quite possibly reverse a lot of the empire's decay overnight if some other dynasty took over (which would be rather strange, as while they'd possibly revitalize the empire to some extent they'd probably not be able to massively improve the empire very quickly).
- While palace coups have a historical basis, I don't think that they'd be very fun to play with as you'd not have a good way (save for perhaps hoping that your Spymaster is loyal, discovers the plot, and you manage to stop it in time) to stop it if the empire's "decadence" rises high enough that the coup can fire (and its rise could, and possibly should, be hard to prevent), compared to at least having a theoretical chance of defeating a Decadence Invasion. I'd personally much rather see a Takeover-style faction becoming available (or just more attractive) if the "decadence" rises high enough (or the emperor is unloved, a child, etc.), or possibly a random prestigious/powerful vassal being given the option to claim the empire and form a faction that is very attractive for other vassals, as that would give you something you actually can (try to) fight against rather than being overthrown overnight because your spymaster missed the plot (or was part of it) with no real option to prevent it.
