the closest you can get is multiplayer co-op on the xbox F:BOS

Well it Could work in a cooperative type of way. Like Diablo online, if
you've played it.
In diablo you can go together or you can go sepperate ways or even fight
against eachother if you enable it. Fallout could definately work like
that... But would still be quite a different experience from fallout 1 and
2.

mmorpgs...
mmorpgs are just so... crap...
It's not that they inherently have to be, it's just that none of them really
gets anywhere beyond the basics of movement, xp grinding and the occasional
player vs player style gaming...
Everything you can do feels like an afterthought... Code that has just been
hacked in to give the player a bit to do...

threaded dialogue? branching quests? not in an mmog, that would be way too
much work, too complex, too heavy load on the servers or some other (poor)
excuse...
Instead you're honored with some repeatable sh.t spawned for you to do,
bring this kill that xp/money grind type stuff which of course very quickly
becomes completely predictable and monotonous...

They're all relying heavily on the community that the game will spawn to
keep players playing... paying.

Building a fail safe server application that must serve thousands of people,
PLUS a game with dozens of features...
It's too big a project I imagine...
Not even companies that can supply loads of funding manages to get it right,
like Sony...

rant over...