On a similar thought, if you could have a system where cities have culture points that are not linked to a specific civ, that would open a lot of possibility.
For instance, cities could generate their own local culture as well as culture linked to their civilization, in two separate pools. If at some point, they have a lot more local culture compared to their civ culture, this could trigger an event where that city (or cities) spawn their own independent civ.
This is what happened to a lot of colonies from England and France after all.