So, SOME DUDE over at the GTA Forums noticed that the Rockstar Social Club site have the above image. It depicts a plane ticket for a "Mr Niko Bellic", bound for "San Andreas" from "Liberty City." Let the speculation begin!
Spoiler? [GTA Forums via videogaming247]
DISCUSSION
@zanzibarlegend: IIRC (and I could be wrong but don't think I am), all of the numbered GTA games have been in Liberty City. It's GTA's home base. So GTA5 will also be in Liberty City.
There will no doubt be other "GTA4 family" games, as Vice City and San Andreas are considered "GTA3 family" games by Rockstar. Where they are set is anyone's guess. I doubt they will continue going to the same well. If there is another Vice City, though, it will not be set in the 1980's. If there's another San Andreas, it won't deal with South Central gang culture.