I dont think that would work with a sandbox game. You would have to create scripted events for the break aways otherwise the whole civil war narrative wouldnt work with random events. I imagine there would be many independent states in some people's games and for some there would be none, let alone the randomness of the time that would happen, for some it could happen right away depending on the situation and for some it could happen 10 years later. So its better with starting 3 - already - independent states that are in civil war for the gameplay reasons.
This is the whole point of a sandbox game. Currently (to my knowledge) there's nothing stopping the imperial factions from being at peace the entire campaign. If the AI is just scripted to do the same thing every single game then a lot of the replay value is gone, and there is no point even having the more dynamic systems like I suggested.
I mean, the Calradians have always been based on Romans. They were the Roman Empire analogue way back in Warband. That's why people were interested in seeing the Empire in the predecessor game. The reason they pull from real cultures is so they can get the associations and fighting styles of real cultures without necessarily being bogged down in real history.
But by making those associations in the first place you are getting bogged down in real history. Taleworlds can't copy practically everything from a real historical state but then pretend that it's just some totally separate fantasy universe when it suits them. When a story copies so much from the real world it becomes inextricably linked.
Here is a good example of why this kind of lazy worldbuilding doesn't work:
When the new Call of Duty came out there was a """"""fictional""""""" conflict in the story in a """""""""""""""""fictional"""""""""""""""""" west asian -stan country
It's just a silly in-game remix of a silly historical event, you might say. But it then becomes impossible to play the game without thinking about the real events that it straight up copies.
Also nothing in Warband suggests that the Calradian Empire is a Roman Empire analogue. It's just an empire that collapsed, and that's basically all we know. (Much of the ingame dialogue actually can't decide whether Calradia was a province in the former empire or if it was the whole thing, but that's another story).
Warband does ride the line of copying history quite a bit, but the reason I don't think it's nearly as bad in that game is because there are factions like the Rhodoks and Sarranids who are unique enough to avoid feeling like facsimiles, and the Khergits and Nords are more like light fantasy stereotypes than historical ones (think Skyrim Nords rather than Vikings).
My point is that Bannerlord is in a bad position where it feels weird if it deviates from history too much, which is hamstringing the ability of the world to feel believable. At the moment I have an easier time imagining living in the Warhammer universe than Calradia, because Calradia just feels more like it's a fake setting created to resemble history rather than a setting on its own.