Things Fallout 4 won't be:
-Set chronologically prior to an earlier game in the series. It's a precedent. Besides, prequels don't exactly jive with the whole post-apocalyptic wasteland set amidst the bones of the American dream.
-Set anywhere outside of the continental US or lower Canada. Again reference American dream, bones thereof. No Fallout Europe, no Fallout London, no Fallout Asia. There are some great post-apocalypse games that can take place in those locations. Russia in particular does these amazingly sell. But they just won't be Fallout without the americana.
So, that said. I could see a couple places that would be pretty cool for the next Fallout to take place -
*Great Lakes wastelands - Detroit/Chicago/Toronto. Old industry, an Enclave citadel, lake pirates, and Niagara Falls.
*Texas, Louisiana, etc. Deep south. Basically grow all the things that made Point Lookout good and expand on them. May be able to explore what happened with the Mexican border after the bombs fell.
*Florida, Georgia, etc. Atlanta, radioactive gators, more deep south, and the ruins of the space program. Has potential.