I think that each city should be a potential civ,
There are three problems and solutions I can see with Revolutionary Nations (which really only exist in the New World, where the Previous invaders were able to wipe out most of the previous nation's culture..other wise they are just an old civ reemerging)
1. How are their bonuses interpreted/given
Solution: Select a playable Civ for that game and Copy them (if that Civ already exists in the game then just change the name... So the Americans might be a copy of the Russians ie Cossack UU, Catherine Leader, but the Name would be Bostons/New Yorkers.. based on the Revolting City they formed from, because the name Russian was already taken by an existing Civ...this is what Civ 2 did, but I'd include the name change so that one could have a couple 100 civs in a game without thinking up 100 seperate combinations of traits/UU/whatever else is Civ specific in CivN*)
2. How they score points (if Final game score is based on your score on all turns of the game, some Civs would be at a disadvantage)
Solution: allow a Civs points to be rated based on when they enered the game
3. Who controls them...It'd probably have to be an AI, unless Human players can 'switch horses' or 'late enter'
*I say Civ N because this is something I would like to be in any Civ and since cIV is probably not going to include this if it doesn't have it already