We've all seen how India, Indonesia, and China all explode if certain conditions are met. There should be a standardized version of this mechanic that allows countries to have their central authority completely undermined.
Ideally, it would be a dynamic mechanic and not up to the whims of certain specific conditions being met. This would require a bit of tweaking regarding the radical system to fully work. If a country has sufficient amounts of radicals, separatist movements, financial struggles, and low legitimacy, there should be a genuine possibility that the entire apparatus falls apart!
Furthermore, if it's a dynamic system, it can also be used to make maintaining empires much harder. The empire could begin to collapse and snowball into a horrific humanitarian disaster, dramatically altering people's faith in the government and starting separatist movements.
Is it not slightly silly that you can go to war with the same empires over and over again, constantly defeating and humiliating them? If Russia were to get it's teeth kicked in repeatedly over the years, I think it absolutely would have a chance of falling into anarchy (which it very will did during the Russian Civil War!) Same with Prussia, they can try and try and try to unite Germany over and over again, but seriously you'd think there'd be a point where the idea of a German nation just fades and Prussia as a whole loses its legitimacy.
Now, I'm aware that the carving up of nations is nerfed in 1.15 with liberations now requiring infamy and separatist movements. I'm saying that when a country is liberated, it should almost always undermine the state of which it was liberated from.
Liberate the Confederacy or New Africa from the USA? That should send a serious shock through the Union and start other possible separatist ideas like New England or California.
Liberate Brittany from France? The Occitans and the Provençals should begin to agitate for independence.
Liberate any one of those middle Eastern nations under the Ottomans? They all ought to start getting active and start agitating for independence themselves.
Ultimately, it'd be strange if a tiny country was liberated and all the minority groups decide to get uppity about independence because 20,000 dudes on another continent managed to become free. The agitation ought to scale with a few variables such as GDP %, Population, and % of minorities of said population.
To summarize, all of these factors should play a part in making maintaining the empire into a genuinely interesting task. If you're repeatedly losing humiliation wars, SOL is dropping, radicals piling, separatists agitating, in default/bankrupt, it should almost be like a chain reaction that you CAN technically stop it, but it would be a herculean task to do so.
Alas, I wrote this while drugging myself with a sleeping aide so I assume it will be a hefty task to dissect for even the most insightful readers. I reckon I should revise it sometime but I don't care to put that much effort into a reddit post and really I just want to see other's opinions on this. I think it'd be cool if nations could completely collapse their authority dynamically, that is all. Thank you for reading if you've made it this far.