Chicago seems like an obvious choice to revisit. Fallout Tactics is non-canon, but plenty of references to it have still been dropped, so it would make sense to make a full fledged reboot that drew from Tactics.

While it won’t happen, I have long supported the idea of a game set in Canada. This is because Americana is essential to the DNA of a Fallout game, which is why Fallout London or Fallout China don’t really work. Canada is different because it was annexed by the US military pre-war. This creates the chance to have locations in Canada descended from occupying Americans who brought their culture with them. But it can be mixed with a Nobel snow covered location and the possibly of actual new enemy types rather than reused supermutants and death claws.

I actually lurked a thread on /b/ coming up with a collective decent first draft idea that incorporated Canada into a wandering war machine from the original pitch of Fallout 2. In the thread’s idea, the rolling machine was actually a mobile vault, which gives an idea how massive it is, rolling through and crushing and destroying what opposes it.