Los Angeles on the Skyrim engine. New Vegas had a Wild West feel to it, but could very well be touched upon.
Combat on ATV's, cannibal tribes, motorcycles, wars for gas, ak-47's, or just bring it back to New Vegas 200 years later with more casinos.
Los Angeles was in Fallout 1. When people in New Vegas mention the LA Boneyard that's what they're talking about.
I dislike the idea of ATVs, cannibal tribals are cool, all gas was used up before the war and so nobody can be fighting over it anymore, AK-47s were replaced by AK-112s in F2, and jumping another 200 years and going back to New Vegas would pretty much change the game to a life simulator in a stable nation surrouned by either land of that nation or other stable nations.