Personally, a Fallout 4 situated in the Midwest region would be pretty cool. As interesting as the West Coast and East Coast were, more happened in the United States, and it would be nice to see what had happened to the Midwest region. Obviously, the central city would likely be Chicago, but you could also visit rural communities that can go as far south as Indianapolis and as far west as Springfield.
However, I would hope for a region in Fallout 4 with an oriental or at least an ethnic mixture (ala Vault 15)... just as an excuse to use the "Many Contrasts" tune again.