I think some of Problems you have mentioned can be solve by pirate and spying idea in this post :https://forum.paradoxplaza.com/forum/index.php?threads/diplomacy-spying-factions-and-pirate.996196/
I don't really want big empires to be impossible or almost guaranteed to fall apart, but I DO want there to be a more than zero percent chance of it happening a time or two in the game.
I remember watching the Stellaris launch stream with the many streamers and having seen one empire splinter into several rebel groups that formed their own factions and thought "cool, so min-maxing everything into science or military might leave you vulnerable".
Sadly, that never really happened in my games and I've played for close to 400 hours now.
I'd love to see it changed so that people that plan on branching out hard and fast would actually have to keep an eye on cohesion and spend some of their beginning stat points (traits, government type, civics, whatever) on keeping the population unified.
Bonus points if there was actually a really good reason to let it happen. Say you are an empire that started at the edge of the galaxy - after a couple hundred years of conquest the farthest colonies in your empire might be pretty miffed at you and your core worlder policies.
It'd be fantastic if - rather than suppressing rebels - you could choose to cede a bunch of worlds to form a new empire that becomes a permanent or long term ally with a massive opinion modifier and/or some nifty unique benefits such as improved research pacts, permanent shared sensors or the like.
You could form the united states of space by having several such subject states that shape their own interior policy, but otherwise closely work together with you as the progenitor empire.
It would be cool if sectors could be released as their own independent nations, and if they agitated for independence, as I believe they talked about early in Stellaris' development.
This would incentivize keeping the sectors both small and happy, perhaps, and would also allow a larger nation to release a custom-shaped sector as a "buffer state" if they so desired, which would be really cool. No border friction penalty with your neighbor on the other side of your vassal/march/whatever.![]()