GTA 5 would be so awesome.
Big fan of the GTA-games, I especially loved GTA 4 because I think it felt more realistic, but New York was just portrayed in an awesome way.
The good thing was the same as the negative, it was just one city.
While the city was awesome, there are so many cities I would love to see in a GTA-game.
For example: Los Angeles, San Fransisco and Las Vegas, yeah I do know that they are all in San Andreas (my favorite apart from GTA 4), but what I mean is that I would love to see them, with a newer graphic-engine, slightly bigger and more "realistic", for what it's worth.
Apart from those, Miami and Chicago might work really nice.
And I don't even think it has to be an American city (even though that will probably always be the case), but big international cities like London or Paris might be cool.