Despite common knowledge, Paradox has attempted to tackle the snowballing problem a few times. The States and Territories system was one attempt at that and I think it worked, but it doesn't really go far enough, and the state limit is a bit too generous. Estates are also probably meant to empower smaller countries by forcing larger ones to deal with some autonomy, but they actually end up being either beneficial or irrelevant most of the time. The penalty for non-accepted cultures is another one. (although this is a little redundant now since you can only accept cultures present in states anyway)
There are a few ways gigantic super-countries could be made less powerful. The most powerful and logical one I've seen in EU4 is a system that gradually increases autonomy the further away a province is from the capital - in MEIOU and Taxes, it's called Communication Efficiency. The idea is that far-flung areas are harder to impose your will upon. It's also not a perfect system but I think it's better than the base game where Ryukyu can make a State in Flanders, get to 0% autonomy, and reap almost as much money as if the Dutch did the same. With that system you can also spend money on Regional Capitals, which are a building that cost ducats in maintenance but act as another "capital" for autonomy purposes. There are also Road Networks which reduce the drop in CE between provinces, and building Docks actually has a purpose now (muh sailors!), since they reduce CE between coastal areas.
A simpler way to implement something like that would be to have a distance cap for creating states. If a state's closest province is x distance from capital province, it's not eligible for statehood. It's less elegant but probably easier to create.
Another change that could make larger countries more difficult to keep together: Scaling the base cost of Stability by your country's development. It doesn't make sense that an OPM and a massive empire have to expend the same amount of effort (monarch points) to stabilize their entire country.
I also think the penalties for having wrong culture provinces aren't harsh enough. Separatism shouldn't just go away then be gone forever, leading to perpetual peace. People should resist culture conversion, and it should cause unrest like Religious conversion does. There should be incentive to spread your culture, and likewise it should be difficult to do so. Right now the AI never converts culture at all.. That means that either the AI just handles cultures poorly, or there's really no point in engaging with the Culture mechanic at all besides "accept whatever has the highest percentage points in the cultures list, then forget it".
Finally, if all else fails, there could also be a generic disaster that represents the collapse of a great empire. Maybe the trigger is having a combination of negative stability, high War Exhaustion, etc, just a lot of bad things, persisting for a while, not being resolved. And it would cause a spike in unrest in provinces with foreign cores, and events causing separatist rebellions. Think of it like the Mandate mechanics except for every country... Because China isn't the only empire that collapses when it's ravaged and unstable.
There are a few ways gigantic super-countries could be made less powerful. The most powerful and logical one I've seen in EU4 is a system that gradually increases autonomy the further away a province is from the capital - in MEIOU and Taxes, it's called Communication Efficiency. The idea is that far-flung areas are harder to impose your will upon. It's also not a perfect system but I think it's better than the base game where Ryukyu can make a State in Flanders, get to 0% autonomy, and reap almost as much money as if the Dutch did the same. With that system you can also spend money on Regional Capitals, which are a building that cost ducats in maintenance but act as another "capital" for autonomy purposes. There are also Road Networks which reduce the drop in CE between provinces, and building Docks actually has a purpose now (muh sailors!), since they reduce CE between coastal areas.
A simpler way to implement something like that would be to have a distance cap for creating states. If a state's closest province is x distance from capital province, it's not eligible for statehood. It's less elegant but probably easier to create.
Another change that could make larger countries more difficult to keep together: Scaling the base cost of Stability by your country's development. It doesn't make sense that an OPM and a massive empire have to expend the same amount of effort (monarch points) to stabilize their entire country.
I also think the penalties for having wrong culture provinces aren't harsh enough. Separatism shouldn't just go away then be gone forever, leading to perpetual peace. People should resist culture conversion, and it should cause unrest like Religious conversion does. There should be incentive to spread your culture, and likewise it should be difficult to do so. Right now the AI never converts culture at all.. That means that either the AI just handles cultures poorly, or there's really no point in engaging with the Culture mechanic at all besides "accept whatever has the highest percentage points in the cultures list, then forget it".
Finally, if all else fails, there could also be a generic disaster that represents the collapse of a great empire. Maybe the trigger is having a combination of negative stability, high War Exhaustion, etc, just a lot of bad things, persisting for a while, not being resolved. And it would cause a spike in unrest in provinces with foreign cores, and events causing separatist rebellions. Think of it like the Mandate mechanics except for every country... Because China isn't the only empire that collapses when it's ravaged and unstable.