I'd like to see one in London, Paris, Moscow, Athens, Rome, Tokyo, or Bejjing, what about you?
You know, I've actually thought about this before. Part of what makes Fallout is the way the world was prior to the world nuking itself: A retro 50's America during the "red scare" when patriotism was at an all-time high, yet heavily flavored in the kind of science fiction they had during that era. The world left behind still has lots of bits soaked in legendary Americana (just look at everything New Vegas is modeled after), and is what really sets this particular post-nuclear holocaust world apart from all the others. Take that away by going somewhere else and you risk Fallout not really being all that Fallout anymore. I'm not trying to sound like I'm biased against any other culture, but some settings (like Fallout) were initially written up with a certain cultural mindset. Let's take Tolkien's famous Lord of the Rings series: He wrote up that fantasy world based on medeival Europe. Now what if a story suddenly came out where Bilbo and Frodo Baggins went to a fantasy land based on ancient Africa? That'd be a jarring seperation from what people originally became fans of LOTR for in the first place. Likewise, if Fallout took place anywhere else besides the retro sci-fi environment of a nuked America, it may as well be considered its own entirely different game franchise.
So, that said, I'd like to see the next Fallout take place east of the Mississippi river again, but this time in "The South". The Florida/Georgia area, specifically. I seriously don't think a single title in all the Fallout games ever released ever went there before.