I don't think they'll ever move it out of the US. It's kind of integral to the flavor of the setting that everything be idealized retro-future and when Americans in the '50s were imagining the "World of Tomorrow," they were basically only imagining the US. Moving to another country would bring up questions like: Does their setting follow that country's view of world of tomorrow, or do they simply mimic the American vision? There is no real right answer here. Saying it follows theirs tampers with the setting that people love from the previous games, saying they mimic the American setting will just upset people who know better.
Anyway, for an actual guess, I think they'll use Texas next. Think about it, it's American so they don't have to explain why the American vision is being used, but it's the land of cowboys, big oil, huge Brahman herds owned by "Brahman Barons" and so on. It's a wild west setting just waiting to happen. Plus there are big cities like Houston, Dallas, Austin, and San Antonio for them to use, so you still have wrecked modern cities to explore. I can just imagine having some sort of mission where you're having to defend the Alamo from Super Mutants or something...