I've never understood why internal stability mechanics are the thing that people go to in the tall/wide discussion. I have always felt the fundamental issue preventing tall play from matching wide is that pop growth has always been tied to the number of planets you have. As long as more planets equals more pops and pops are the fundamental unit of power, it will always be true that more planets is more powerful. Trying to introduce all these different ways of holding it back is missing the point and isn't going to result in a balanced game.
I think the fundamental change Stellaris would need is to decouple your empire's pop growth from the number of planets you have so that large planets can grow pops faster. In theory, they have that in one place with the ecumenopolis's boost to growth, but a 50% additive increase on one or a few planet is insignificant compared to adding another full planet's worth of base growth to your empire (also, I've always felt like having more than one or possibly two ecumenopolises greatly reduces their impact in the same way being able to spam Dyson spheres in the old days made them less compelling). What we need instead is to add options for developing your planets in a way that increases pop growth in a fashion similar to expanding outward and adding more colonies.
I also think it's rather silly to set up all the mechanics so that being large in the tall direction means you avoid a bunch of penalties and other mechanics while being large in a wide sense has to introduce all these penalties. If they execute rebellions and internal stuff well, I'd want to see it and interact with it even when I'm a smaller empire. It should be different, sure, but it shouldn't just not happen.
i think being able to increase pop growth when you ascend a planet in some way could help. in theory, playing tall you should be able to ascend a higher percentage of your planets, and be able to ascend them higher. but i do think the unity system interacting with stability mechanics would also help. like, running a unity deficit for example should eventually start to impact your stability. but beyond that, decisions made in the upcoming political content should have an impact on your rebellions, not a linear one but gradually till you reach certain tipping points, then gradually again. like punctuated equilibria. anyways, what if the crisis mechanics were used for rebellions for example, only instead of crisis points you're accumulating social crisis points lol. see what i mean?
like if you get to a certain point of oppressing your people in terms of the decisions you keep taking or of oppressing a crushing majority of xenos or slaves or whatever, a fanatic faction is born. these can then grow enough that there's another tipping point reached, and a reskinned crisis mechanic for a social crisis opens up. and then after that, that's a big flashing warning sign of five stages or whatever that you kind have to be really intentionally pissing off your people or whatever to not notice. then you have a revolution, again still not creating a new state because that's really imo rare unless there's outside interference, you have a certain like ultimate crisis and if you lose that then you start losing planets but still not to a new state. it's like a situation of dual power. hell they could use the same contested map paint filter they had from back when you could build in the same systems.
anyways, basically at that point it starts becoming more a civil war, and the worse you do the closer it gets to either your leader being killed and player forced to adopt new ethics "by the masses", or if the player managed to fight the rebels to a standstill where they have strongholds and so does the player, and they've reached the civil war exhaustion meter of the population,
then the player loses territory. see basically it would be between regular wars that something like this could pop if you're taking certain actions or playing certain ethics. and don't worry, the same mechanic can go the other way.
say i'm playing fanatic egalitarian with utopian abundance. now say i have a bunch of crime popping up. that's a black market, outside of my egalitarian economy (1:1:1 consumer goods per class), filling someone's filthy pockets. that's basically at that point very tempting to certain people in the bureaucracy, to have their palms greased to keep the capitalist black market rolling in filth. so if i choose to ignore that, i risk something very dangerous, elements of my bureaucracy want to get rid of this economy because as Trotsky once said a corrupt privilege is only half as sweet if you can't give it to your children (paraphrasing). so they want to be able to accumulate vast amounts of capital themselves, get rich themselves, adopt something like decadence perhaps? anyways, let's say these folk want a return to authoritarianism, to live it up on the backs of the specialist and worker class. maybe the shadow council civic could also play into this!
if i let them grow long enough, their faction which wants to seize power will also grow its support (what if ships and their leaders could acquire ethical allegiances at the fifth stage just before a civil war? warning you of the need to clean house, like Allende was warned). And all the time decisions new chapters of the social crisis are opening up, both in a social crisis system thing and in a thing similar to the digsites but for political events. and if you take the right decisions, disarm the opposition or whatever, and aren't hamfisted but also aren't naive, then you're fine. but if you don't act swiftly and firmly, if you ignore the rising authoritarian threat, even a democracy can die without a fight. which isn't the end, but if there is no fight than the democratic faction is left with a weaker underground, so if the player wants to roleplay overthrowing that dictatorship in the late game, they better go down fighting and leave an inspiration for the next generation
