A Fallout sequel, for me, is a game based primarily on Fallout 1, the Fallout game. There is no equivalence between it and the others, by a variety of reasons.
For starters, I don't like the idea of an ever expanding canon, specially when it's been passed around by several companies and project leaders, writers, etc. Only the nerds who hang around tvtropes and the fandom wiki pages like that sort of thing, the pointless accumulation of trivia, backstories, etc. It ends up trivializing everything.
I enjoyed F2 and FNV, in some cases more than F1, but ask yourself: do we really need the Legion? If San Francisco ends up being depicted again, wouldn't it be better to just completely forget about F2 and make it from scratch?
F2 is a thorny issue, because it obviously feels much closer to the source. I'd say half the game fits seamlessly into the world of Fallout, while some things could suffer a few changes (e.g. New Reno), and a few are irredeemable (e.g. SF, talking Deathclaws).
FNV I see as a spinoff, a very good one, but only a spinoff. The setting itself was fundamentally different, showing a civilization well on the way to recovery. It's much more focused on power politics and ideology. All that stuff is interesting, but the further you go with it the more it obscures the original setting and its purpose.
