Below is the post I made in another thread, and to address some comments I think outside America wouldn't allow for the Fallout vibe, so It would be a game about the post apocalypse, but not a Fallout game. Like the people that mention the U.K. as an option in threads like these, there would be no Power Armor, none of the factions that have been in the game, so many things would be different enough that it just wouldn't really be a Fallout game.
I think the smart move would be the Pacific North West.
I could say Seattle, but that would be limiting.
It would allow for some Canada exploration, perhaps via a DLC, and from a real world point of view, having a satelite office in the northwest would conveniently give them a pool of software development people to hire.
It's coastal, has a few military bases in the area, a rainforest, and again, it's on the border with Canada.
If they emulate reality it would have an aerospace company that built the planes that dropped the Atomic bombs on Japan, and a software company that, without some slick salesman competitor with a mouse, would go on to make the norm a comupter that wasn't stylish or colorful or all that hack proof.
If they want to keep being able to reference the atomic age and the bombs, and have some leeway in story with the altered reality of an American annexed Canada, then it's a good place to go