Doing VC again would be plain stupid. Of course I'd buy it, but I wouldn't be happy with the choice of the city. And by the way you can't really say that you'd want to see what VC evolved to 30 years later. It's not the same universe. LC on the PS1 was New York like. On PS2 it was vaguely east coast, as the Housers said. On PS3 (And whatever else) it was almost a carbon copy of New York.
No, what I really think it will be is a game called GTA: New Austin. It would be VERY much like Rockstar to make Red Dead Redemption set in the very early 1900s, and then making the next GTA in the same place, just completely different. That's what I think they'll do.
I'd almost take anything over VC again. Now is their chance to make something new and different. They shouldn't re-do the old cities. It was a good idea to do LC in GTA4. Every GTA series has started there. Thus also the next-gen games has started there. Good idea. And in the words of Monty Python: ...And now for something completely different. Now they have to do something new. The time is NOW!
Actually, as I stated before, GTA: New Austin ain't a bad idea. Neither, btw, do I think that a Los Angeles based game would be bad. San Andreas was a whole state on the PS2. How about making GTA: Los Santos? That's not a bad idea either I think. Think of it: In the 3D GTA games there's not been a LA based one (Like ONLY LA). Wouldn't that be great? It's got gangs, it's got Hollywood. It's got it all man (Like London do too, but LA and New Austin is a better idea I think). It doesn't have to be the hey-homie thing like in GTA: SA. It could easily be something else. In fact, my dream would be the L.A. area in the late 60s/early 70s (With the surrounding territories of course. Never again do I want a city-only game like GTA4. It got boring after a while).