Just to be clear im not talking about interplays attempt at a online game either
I think it could be fun sorta like a massive free roaming post apocolyptic version of multiplayer online shooters. Only instead of it being on small maps, it would be the entire waste. Make your own fun sorta thing. Could have different servers. One could be a "hardcoe" style where when your character dies he dies permanently (like diablo). One could be a more friendly online type one where other players cant always kill you whenever they feel like. probably some that lie in between.
Probably wont be able to use vats vs players either.
I dunno, the entire time ive been playing fallout 3 I kept feeling like I wouldnt bat an eye if I met another player out in the waste, that it would feel largely natural. I think it "could" be awesome.