This may or may not be exactly what you're looking for Tholish, but have you checked out the Revolutions Mod? I love that one, it has a very dynamic use of cities breaking away, old civs re-emerging, etc. And you can easily tweak all sorts of things, including how likely it is that a city will break away. And lots of people have already combined it with various other mods, like RevDCM
). Each Ideology would grant its own sets of bonuses & penalties if you adopted it as your State Ideology, & I was also having certain Ideologies make certain civics choices unavailable.

