I bet the corporations in fallout sold products outside the country, which could mean a lot of similar elements contrasted by differing environmental elements could make anywhere else in the world an interesting setting.
Though if it were to be in the US I'd like to see one done in the midwest, like maybe ranging from Milwaukee/Madison to the north, St. Louis to the south, Indianapolis to the east, and I dunno, somewhere in Iowa to the west, I think a map of that size would wind up feeling about the same size as the map from fallout 2, and provide a lot of variety between types of post-apocalyptic hellscape. The plot could be massive as it'd be a logical place for a standoff between tribal forces, Legion, both sides of the splintered brotherhood, and it'd be easy to imagine Enclave involvement there as well. You could easily create homebrew factions for the area as well. It could be awesome to play a fallout where there is no clear protagonistic faction, they all offer something at the cost of their ideology, can be allied with or played against each other, and all from the perspective of the wanderer who sees if someone doesn't do something, the tension between the forces will ultimately result in a massive 8 or so pronged war.
If this was taken down a really inventive road let the player try to engineer peace or war, alliances between factions, as well as cold war-esque black ops like concentrated strikes by "radicals," espionage, and sabotage between factions. But based on the players actions maybe there won't be a massive climactic war, or maybe there will and the factions will utterly obliterate each other, or a few will ally and win the war against the splintered ones, or whatever permutation could occur at that point. Gotta..Stop...Rambling...sorry.