A future GTA would be awesome, and really I think they should just remake GTA:2 and give it a different name. I never really liked that game, but I think that variation of Liberty City would look just awesome on PS3.
I think aside from the future GTA and a present one, taking GTA to the south would be awesome. Similar to SA, they could have one big fictional southern that has all it's most lively cities or unique combinations thereof, Dallas, New Orleans, Atlanta, Saint Louis, etc. Hell, I'd just settle for New Orleans to be honest, can you imagine all the next gen boobies from a Mardi Gras celebration? :lol