Exactly. I think the hard cap on fleet size in endless space seemed very arbitrary although it kind of worked. It would not work in stellaris simply because of the scale of the game that constantly grows while you play it. Think in the end the best way forward is probably an approach from multiple sides:
- At the same time space battles itself need to be reworked somehow so that a smaller fleet can still destroy some ships in the larger fleet and not be flat wiped out if they are less than half the size. For example ships could be forced to keep a larger distance between each other always and most weapon ranges could be reduced. I do not know if the first point could even be implemented in the given engine but it would certainly give smaller fleets a big boost because parts of a doomstack could not even fire at the enemy.
- Sectors reacting positively to fleets being present in the sector and negatively to no fleets being present (undefended). There needs to be some economic downside if you move all your ships out of your empire.
- Buffing stationary defenses significantly. For example early game a spaceport is a significant threat but very soon it becomes irrelevant. I feel like spaceport levels should severely buff the combat strength of a spaceport. At the same time military space installations should be buffed alongside. I would not mind seeing a lategame space fortrest @ 20k+ strenght. Of course they should take longer to build and be more costly but if all types of bases are stronger you can always take a defense platform lategame if it still gives a few k in fleet power. And for the love of god remove the limitation on stacking space installations (remember that episode of DS9 with a large federation fleet attacking defensive satellites?).
- Adding new means of system defenses: minefields, very long range weapons (even a doomstack looses ships then)
- Giving more control over occupied planets, i.e. being able to transfer/purge pops, loot buildings while in control.
- Making warscore react more strongly on actually occupying wargoals/capitals (ticking warscore?) and less on unrelated systems (see EU4) to increase the importance of holding specific systems.
- "looting" the enemy economy with every enemy system in which you have a fleet (effectiveness dependent on fleet size, see EU4 fleet blockade)
- About cap is what im trying to tell. We need some way of limiting the power at the start and unlocking it with our actions. (Tech, supply system, etc) . This will go a long way of limiting doomstacks early/midgame but they should not stop us from going crazy by endgame where the real threats like the unbidden come out and demand from us those crazy fleets.
Here is more or less my vision. Im thinking here in 4 phases. Early game (corvetters, maybe a few DD´s). Midgame (mass DD`s, few cruisers). Mid to late (Battleships are very rare if you have them). Lategame (battleships are normal).
A) Early game -> should have very few doomstacks. You have very few key systems and ways of producing ships. DD is the best you have IF you have them.
So it should be much more about taking new colonys and destroying outposts than taking the enemy capital. ITs possible but should not be easy or even possible unless you get him offguard.
Lets say you can reach around 2k fleet power. Enough to defeat defenses + Stations alone but not Fleet + Station + Defenses. For that you need to lure the enemy or have a superior production to win by atrittion not allowing his defenses to heal/rebuild.
B) Mid game -> where we unlock cruisers and DD`s are normal. Doomstacks should start to matter and appear, but not everywhere. Border colonys and neutral space are still problematic to bear the full power of a full sized fleet or to stack them. Around your core and more developed worlds you can start to stack fleets and have very huge defense systems to stop this kind of strategy.
Taking empire capitals and core worlds is now not only possible but kind normal. But taking the "core worlds" of a empire demand some investment in a supply chain to allow the enemy to pierce heavy defense systems or the enemy doomstack. (or worse, both).
By the end of this phase you can reach around 10k fleet power. Defenses can match that firepower but they take resources, lot of time and upkeep. You can start to overpower them with 20k or even 30k but you need to "prepare the field" first by allowing yourself to send your fleet there. (You can´t Snipe a coreworld. You need mount a supply chain).
C) Mid to late game -> you have battleships and now not only your fleet size increase but you can start to walk around without much problem almost everywhere. Going to the other side of the galaxy is still a problem. But you can safely reach everywhere in your borders and project power up to a point with little effort.
here war starts to get bloody. To defend from that turtle empires really need to invest in a mix of Planetary + Station defenses if they really hope to stop a stack of big fleets. (Plus sending their own fleet). Wide empires while they have resources they will not have time to make those kind of defenses everywhere and will have lots of gaps to exploit and make a dent or even survive long empire incursions.
Reaching 50k is possible now. Maybe even more. But not everywhere yet. Fallen empire are still a threat as they can bear their power everywhere. Leviathans are a threat to wide empires as they can target new conquered worlds and defenses.
Defenses can reach 50k. But outside of your "Shipyard system" its not recommended. IT will cost a lot and it will take too much time, take planet slots and cost too much upkeep to defend more than a couple of systems with this.
Around 100k should be the theorical maximum for this part of the game. There is no unbidden or AI yet to demand more firepower than that.
D) very late game -> 100K to infinity is the normal here. Defense systems by themselves can´t defeat those monsters. it gets so expensive to defeat this with a stationary defense system that is simply not worth it or the empire is not able to pay the upkeep. (or the entire planet becomes a fortress with no farms, energy etc. Only guns in all 15-25 slots plus stations on orbit becoming a living Death Star). (But would be fun to see some very large empires having this kind of chokepoint

. And much more fun to breach this ^^ ).
Unbidden and lategame crisis will come here. There is almost no limit to what you can bear and how many power you can project. Only some moderate limit if its on the other side of the galaxy and only if there is nothing there. (Lets say 1000 galaxy, you are on one side, everyone is dead or unwilling to allow your travel to the other side and you want to "snipe" a fallen empire planet there, you can´t send you 1M fleet so far. You have to work with lets say around 500k, by the half of the galaxy you can bear something like 800k. ANd you have
no limit in your border or very close to it.
it will look almost like the game is now. Only difference is that defenses bite and most empires and federations are very huge with lots of targets. So going doomstack will work but not really desirable as you have too many places to look.
About the other points.
-> Totally agree with point 2 that smaller fleets should make a dent on bigger fleets before dying or fleeing. Increasing the time the fight takes is kind of mandatory for this to happen.
-> Agree, but remember with the game right now you can reach to 14 systems with little effort. (3 + 2 burocracy + 2 colonization tradition + 5 ascendancy project + 2 midgame research). Plus some more lategame. So in a medium sized galaxy this is more than enough and you will only have a very small sector that will demand a very small fleet.
But i like the idea of planets demanding stationary fleets (or defense systems) to keep themselves happy. At worst you combat the doomstacks with upkeep.
-> Spaceports for me need more defense slots. To add more weapons, shields, armor, etc. So you can choose making that spaceport a Dockyard, A farm/Research center or a battlestation.
Defense stations + Planetary defenses should be able to always match anything you can bring, unless its very lategame or the planet is small and you don´t want to turn the planet into a deathstar. You can balance this with....
- Time (Defense taking really long time to build, months/years so if you destroy 1 station by the time you come back he will not have everything up again).
- Upkeep (if you have a 50k defense, it will cost the same or more than a 50k fleet, this alone make sure you will only put a super defense if you really need it).
- Resources. (If you spend 500k minerals to make a 50k fleet a defense like that should also cost around 50k minerals).
Let small empires turtle themselves and make a wide empire bleed for every system they try to make. Its always fun to breach "Troys" or a " Battle of thermopylae "

.