I'd say London or some other western European city (Adding an international flair and larger perspective of the Fallout Universe), Moscow (Massive, Iconic, unique environment and a lot off ways they can be creative), Nagasaki Japan (Irony), Buffalo NY (I live here so I have a bit of bias but even now the cities been severely depopulated and is LOADED with vacant industrial and residential facilities, perfect if they want to create a more super mutant or raider based experience), NYC (The largest and most iconic city in the US and arguably the world), New Orleans (Different swamp environment, the mississipi river, iconic and for a twist could still be capable of producing gasoline because of the oil refiniries) , Atlanta (Practically the capital of the south), Chicago (Iconic and one of the most populous cities in the country), Honolulu (Tropical island twist on armageddon), Miami/Cuba (with half of the game in each) or Boston (Because of its location as the heart of technologically advanced commonwealth).