I would like to see the Pacific Northwest. There wouldn't be any of the known factions from the other games, with the possible exception of the Followers and some Enclave that survived the oil rig and their kids. Other than that, it would be a blank slate. All new factions would have to be created to fill the void. I'm imagining the other groups being too small to be big players in the region. So no massive Enclave army, I was thinking a few outposts or a city state built on top of a control Vault.