An European landscape would be a very cool addition to the fallout 3 franchise. I mean, just imagine, you can have countless add-on cities, not necessarily related story wise, but just a glimpse into what happened some place else other than the U.S.
It can start out with London, Paris, Madrid, Rome... and then, la piese de resistance, Moscow. Just think about how many landmarks are there, the Kremlin, the Red square not to mention St. Basil's Cathedral and of course the fact that it would be the other side literally.
And while we're on the subject, why nobody mentioned China... I mean the Chinese invasion was everywhere in the Capital Wasteland... why not a little change of perspective