I'll throw a guess in
Given that previous GTA games have all been set in distinctive "American" locations with names that refer to the inspiration without citing it directly, I'll guess
Grand Theft Auto: Alamo
I mean, once you've got the east coast / big city, and you've got the west coast, and you've got florida, what else is distinctively different in the US? GTA: Olympia? GTA: Bayouville? GTA: Smoky Mountains? GTA: Cape Cod?
Whatever the setting, they'll have to speak English, and it will have to be distinctive so if it's not London, I'll guess Texas.
Or, heck, maybe they're going to go all Assassin's Creed on us and it'll be on Mars.