I'd really like to revisit Vice City, but as long as it's not an obnoxious 'herp derp im so gangsta' game like San Andreas, I'm okay with it. I'd also like Rockstar to take a few notes and take a few steps back from IV, it was a detailed world, but the giggly humour was replaced with a more stark humour, which was never GTA's style. (Barring 1-2 moments in previous games)
They've done Vice City and Liberty City twice each now and I'm bored of revisiting the same places over again.
I'd like it to be London, but it's going to be San Andreas again. R* were hiring voice actors for a game set in modern LA a while ago, the only other thing it could be is Midnight Club LA 2, but the characters seemed more like they'd fit in the GTA universe.
London has been done already as well (PS1) and I hope its not San Andreas again, it would kill the original game I loved :/
Since the games can hold more how about they try making the whole of the UK, since they originated here there could be a bunch of cool easter eggs and their HQ in Dundee (IIRC)