well, fallout: chicago has already been done, though a revisit tie together the stories of the east and west would be interesting. i wonder what the canon ending would be. i think it could be pretty interesting if they had the canon good ending, but then the ncr comes and starts to mess things up, while the DC brotherhood makes a return, and simultaneously trouble brews from within (some kinda separatist group from within the chicago society. cannot remember any names atm since i never played it and only read the story once or twice).
could be an interesting clash of ideologies. everything depends on what the canon from Tactics, New Vegas, and 3 are though. still something i would play.