I wouldn't mind a Fallout spin-off that's maybe a first person shooter set during the Pre-War era. Though, if they ever did make a game like that, it'd be probably be set in the same time period as the rest of the series, because of the oh-so marketable Brotherhood of Steel. I just think some of tech in the series would be interesting for a straight up FPS, but I don't think it would fit in with the timeline if took place after the Great War. That's just me though, and I really wouldn't want the main series to become multiplayer. I don't really think RPG's usually benefit from having multiplayer.