Dont we already have an official Fallout 4 Location Suggestion Thread?
Well, his topic is if Obsidian makes another game, not FO 4. So, in fairness, it is another idea, however, the new FO location thread topic was renamed to future fo games, so technically, could also fit there.
To address question posed by the OP, I wouldn't want obsidian to only focus on the west and beth the east. I would like to see how obsidian can do in other areas. This kinda depends on where FO 4 takes place. If FO 4 was Detroit and Chicago, I would like to see perhaps a Colorado, Denver or perhaps Texas, st. Louis, or new york setting from obsidian. If FO 4 someplace else, then I would like to see obsidian do chicago/detroit.
As far as factions, the followers, taking a more aggressive role, bos, enclave, slavers. New factions depending on area.