I was thinking midwest too, maybe Chicago?
Technically, they already explored the Chicago area via Fallout Tactics. Not sure how canon that game is with Fallout 1,2, and 3 however. Either way, I wouldn't mind seeing a return to that area(especially if they expand on what happened in Tactics).
One of the problems I see with Fallout moving to the international scene is that there wouldn't be a lot of the Fallout "staples". For example, there would not be very many(if any at all!) suits of Power Armor as that is an American design. Also, many of the robots would not exist for the same reason(Mr. Handy, SentryBot, Robobrain, etc.). Would the Super Mutants be over there or would the FEV be unknown? No Robco, no Vaults either.
It would be really cool to see various international landmarks in the Fallout world, but I'm just not sure if it would work. :unsure2: