I think a Planet of the Apes style society would be cool, although I’m kind of on the fence as to whether that actually fits well into Fallout, or if it’s maybe too derivative of, you know, Planet of the Apes. Do you think they’d keep humans as slaves like in the movies?
As far as environment goes, maybe I’m just old fashioned but I’m of the opinion that most of the world should be hot and dry after the war. It might seem bland and unoriginal but I like the idea of lush regions like the PNW becoming desert wastelands, it really drives home how destructive the Great War was to the planet. To bring up Planet of the Apes again, that (original) movie takes place around New York but the environment looks like desert/savannah.
Giant bison sounds cool, I would imagine they would’ve gone extinct considering that they’re already endangered before the war (and I can’t imagine the pre-war world was too concerned with wildlife conservation), but it could be an interesting irony that buffalo have adapted through radioactive mutation to roam the plains of America once more.