I would definitely like to see a return of vassalage, along with colonization on different continents perhaps leading to revolution and independence if the loyalty and happiness issues are bad enough on the distant continent.There are still old mechanics from Civ IV that haven't been adapted, like Corporations, Health/disease, and vassalage.
But instead of just becoming "Free Citizens" who attack everyone, they could become some kind of greater city state, with multiple cities, which could interact with the entire world.