That could work. Maybe expand it out into the Caribbean a bit, with a trifactory of powers (Cuba, S. Florida, & a unified Haiti/Dom) vying for control of the region. Perhaps introduce ocean travel using salvaged pre-war junk ships powered by either improvised wind sails, slave labor, or very rarely maybe gasoline or fusion generators. That'd actually be kind of bad-ass to have a modern-day pirate (like, in the Somali sense) setting with a splash of the romanticised privateers of the of 18th century.
Salt, specifically, has been one of the most important natural resources for the building and flourishing of civilizations throughout history - and it never comes up in Fallout games. In a post-war world that lost it's ability to industrially extract and refine salt, it would once again be worth it's weight in gold (er, caps) and would drive trade… and conflict. The Caribbean would be a great location for salt refineries to crop up, and the sea-lane trade routes to the mainland US and S & C. America to be heavily contested areas by regional powers.
The shift in tone and (hopefully, gameplay mechanics) would be a huge departure for the series though and might be better left for a side-game.
Australia would also be a fun location, because it's already fairly wild and hostile. There would be a hell of a lot of room for creativity in monster design too, considering AU's unique fauna - damned near all of which being poisonous.
But most of all, I just want a location that's suitably overgrown with vegetation and looks like it's been ruined and suitably abandoned for ages.