The GTA games love to play through the stereotypes. We've done the faceless white guy with no real affiliations, the italian mobster, the black gangster, and now the russian mob (the last three pretty much being the "cool" gang images for their respective release years). If they do something different, my best guess for the next character would be a Yakuza, and so I'd expect Tokyo as the location (playing in the U.S. as a Yakuza just doesn't have enough power to it). It could even be a prequel (Liberty City stories style) to an existing character in the universe.
Unfortunately, we're probably more likely to see a repeat of a prior title (shock horror!). San Andreas could be up, after the relative success of both LC and VC Stories and the new potential of the next gen consoles. Hell, even a GTA: London reimagining into 3D could be possible, what with The Getaway 3 being canned, although that's a little less likely.
Australia is no chance as we don't have enough of a superficially identifiable nature (Canada, America, England, etc - all pretty much the same on the outside lads, just admit it). L.A. Noire is already going to cover the '40s option, and a "Wild West" is just not going to happen - it's Grand Theft AUTO, not Grand Theft Equine (yes I know there were cars in that era, but not widely available, and winding a crank before stealing one is just too much work). Las Vegas was already done in the 3rd island of GTA: SA, Miami and the 80's has been covered, and the future would be too much like a bad joke - it works well for RPGs or FPS, but silver suits, aliens and UFOs don't play in a criminal sandbox game (at least without a Star Wars backing).
If they wanted to surprise me it would be a 3rd world country - South America, or perhaps even Africa. Rising from slums to the elite, working through drug cartels and militia, dealing with a despot. But the political commentary could render that impossible to do well.