Tucker from RvB
Chieftain
Okay, I got this idea from a game called "Civilization: Call to Power". In that game, when citizens in a certain city became really unhappy, the city would break off and create their own civilization that would behave just like all the other civs and it would keep all the tech that you had researched. The only problems with this is that it would be very easy to instantly invade these cities again, but perhaps this could be compensated for in some way.