Would it make the player Jarl of an existing city or would a new city be created? There would be a lot more work that would need to be done, but I think that creating a new city would be really cool. It would take a really long time to make, though.

You could also figure out a way to make the cities go to war and form alliances, ultimately resulting in the player gaining control of all of the cities of Skyrim. Personally, I think that what I just suggested might take away from the overall game, but some people out there might be interested in city warfare and sieges, like that of Whiterun.