Absolutely nothing has been confirmed about GTA V, beyond there being a trailer on the 2nd November. But that's not stopping the speculation machine from grinding its mighty gears. And the most convincing rumours seem to be coming from Kotaku.
Apparently the long-running rumour has been that the game will be set in Los Angeles, and thus incorporate a Hollywood theme. This would coincide with the money-like appearance of the game's V logo, suggesting ritzier, less grimy areas for the game. And of course Los Angeles contains places like Skid Row, Compton, and South Central and East LA aren't exactly tourist destinations. Some are further speculating that it might be the real LA in the game, rather than one of Rockstar's meticulous facsimiles, but I fail to see what advantage there would be to this to the developers.
The other rumour the big K are putting out there, they say heard from multiple sources, is that the game will feature more than one playable character. Which would certainly be a deviation for the series, since the first two games. Of course, such a rumour could be generated from the developer crafting extra characters for the additional content, as we saw with GTA IV, and the following Episodes addon. (Kotaku try to justify this with some rather spurious nonsense about how numbered GTAs show the most significant changes in style, as if San Andreas weren't the most different of the lot despite not having a number attached.)
CVG throws in the belief that the game will feature "a Welsh monk/cult leader/yoga teacher, a weed evangelist, a swinger and - top of the pops - an ugly but comfortable mellow Californian divorcee."
I think it's much more likely the game will return to an expanded Los Santos, perhaps even with San Andreas around it. Although clearly the only sensible setting for a GTA V should be Anywhere, USA, three weeks into the future.