I'm gonna say one of a few places.

1) Fallout: New York
2) Fallout: New England (with Quebec and southern Ontario)
3) Fallout: United Kingdom
4) Fallout: Southwest (Mexico, Arizona, New Mexico, Texas, Southern California)
5) Fallout: Korea

Discussion of future Fallout games belongs in the Fallout Series Discussion forum, and in fact there's already an http://www.gamesas.com/index.php?/topic/1076417-expanding-the-fallout-universe/ in that forum so let's continue this there.