Well we know for certain that some factions will appear from rebellions against others, we've already been told Scotland the the USA will do this.

However, I really doubt they will be playable. Well the USA will be playable because they said there is a campaign for its War of Independance, but you probably won't get to go factions appearing in mid-campaign, as cool as if would be.

You can choose which side to take in a civil war though, which IMO is going to be fantastic. Cromwell's Republic will be restored in England!