Fallout takes place in Southern California. Fallout 2 takes place in Northern California, Southern Oregon and Reno Nevada. Fallout New Vegas takes place in the Mojave desert/Las Vegas area. So there is no need to go back to those locations.
New Mexico and Arizona are Legion territory but it could make for an interesting location but I don't really want Bethesda messing around in the West and they seem to want to focus on the East.
Chicago is my pick for a location. But that being said there is already a thread for future Fallout locations here ==> http://www.gamesas.com/topic/1501546-official-future-fallout-locations-suggestions-28/
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I also really hope for different regions of the US to start getting their own games.
Like Styles, I really want a Midwestern Chicago focused game.
I also think that the Pacific Northwest would be incredibly interesting.