Silly suggestion; Fallout: Gothenburg. :3
Like a reversed settlers-story, a pre-war freighter is used to carry emmigrants from the scorched wastes of the American east coast to the ever so slightly less scorched bits of Europe. Topple the king, or rule from the shadow of his throne! Make the old world your new world! And watch out for radioactive mackereel.
Proper suggestion: New York, or perhaps Boston. I did like the urban enviroments in F3, as well as the suburban/countryside parts. While I really did like the enviroment in New Vegas as well, a place that has always been (more or less) a desert isn't as interesting as a temperated zone turned desert by the war, with burnt treestumps and whatnot.
While I do agree that Fallout-games would work best in America, though, I still think it'd be interesting to explore the ruined Fallout universe from a different perspective at least once. Perhaps London or Paris would be a close enough compromise?
Both have an interesting Metro-system (and Paris got the famous catacombs, to boot), well-known landmarks and would look great in post-apocalyptic make-up. If not fit for an entire game, maybe bits of them could be used as DLC-material? There ought to be at least one loony out in the wasteland with a functioning zeppelin or something.
(And in before Frankophobia: the United States and France are way more similar than anyone cares to admit. Embrace your brothers in stripy shirts/with cowboy hats!<3)