No Liberty City...not in the coming 30 years please. GTA 3 was fantastic, but GTA 4 was a bit disappointing. Infrastructure was a hell, all concrete, a Russian guy. No, that didn't do it for me. GTA 5 is awesome. Only played 4 or 5 missions, but I can't get enough of getting chased by the police thrue those mountains, and discover area's every day where there is a lot to do. I can't even fly now and I think this is the best game ever made.
GTA 6 should be in Vice City in my opinion. Based on the '80's style. But I have 1 problem with that: Mountains...there are no mountains in Vice City.
Niko Vercetti 112 suggested San Fierro '70's style. I totally agree with him. Never done the '70's and San Fierro properly sounds great. Chicago would also be great. I'm from Holland and even I know Chicago doesn't look like NYC at all. Especially if they do Chicago in 1931 style. But then we would have a car problem. There were a few and they would have no speed.