If they take it out of the U.S. it will be even less of a Fallout. Fallout is pretty nationalistic. Apparently your character is the savior of D.C., so it won't be there either. My bet is that it will either move back to the West Coast or the South-West like Van Buren was planned to be, in which case they will be STUCK with the canon and will have very little choice but to move from there. It's pointless to speculate on them taking it outside of the continental U.S., because, well, that's a stupid idea. At first they planned to carry on the story from the West Coast but they changed their minds, which makes me think they will head back that way in the future. I think the only reasons they set it in D.C. this time around were:
A. They live there
B. They wanted to write their own story without stepping all over the events of the previous games
C. They wanted a chance to test out their version of Fallout where the stakes weren't so high, so they moved it to a place completely unrelated to the other two games so they could basically worry more about the technical aspects of what they were doing than the story (which is obviously what happened)
However, I am optimistic about Fallout 4. Fallout 3 could have been a lot better, but it's a least a very good start for them to get some of the technical and aesthetic aspects in place. Maybe next time they will actually move the plot forward instead of moving it sideways and giving us a recap of everything we already know.