As I've posted in more elaborate detail on a gaming forum I go to, I hope they return to Vice City or San Andreas set in present day, even if its not this game. The changes would be very interesting especially if GTA4's remodelled Liberty City is any indication. Plus, it would just LOOK incredible on current gen graphics, and I'm not even a graphics kinda guy. But given the money theme they seem to be going for, I'd guess Washington.
As long as they don't go back to the monotonous Liberty City, I'm fucking excited. And for those who didn't think GTA4 was much fun, I agree in a way, but I suggest you play the Ballad of Gay Tony DLC. The missions in that game let you have much more fun, it was what GTA4 should have been imo. It was almost as over the top in terms of gameplay as the Saints Row games, only without the level of customization.