In the real world? In an Obsidian Studio.
In game, I would either enjoy the heck out of:
A port town on the Great Lakes or even just a normal town near them. Sault Saint Marie, Duluth/Superior, or even Mackinaw. The frigid weathers would finally mean a nuclear winter setting, replacing the desert thing they have been doing since game 1 with something new. Creatures such as mutated fish, moose, and wolves would be awesome, with a focus on more hunting gameplay at times (The creatures hunting you for instance). More hunting and civilian weaponry then military, with a lot more scarcity. Huge industrial districts, worn out and half sunk freighters, and the fact there would be tons of vertical gameplay since they all take place on hilled cities.
Or perhaps Washington (the state). Wild woods of the area overgrown for centuries. Human contact limited at times. Imagine a part of the Fallout world nature has almost fully reclaimed, and seems empty and desolate in a new way. Vines and trees jutting out of buildings, tiny towns being eaten up by the mutant force of nature. Constant Radioactive/Acid rain would force most humans underground or in cover, making nature itself a constant threat and gameplay element. Perhaps Fallout Seattle would be a huge hub of skyscrapers and tech as well.