Well co-op for up to two players could be fun, roaming the wastes with my friend Sorta like saint's row 2

Without the gamebryo engine,

Maybe

no pvp, but definetly something like a trading post to buy/sell from other players.

No.

Mostly because the devs have to dedicate their precious time on MP as well, which means less time on the SP. Just look at COD or MOH (or any other shooter) what that delivers. Short SP campaigns (although they can be entertaining)

SP is the strength of an RPG, keep focusing on that.

Perhaps an on-line meeting place (situated in a major city or whatever) where you can hang out and/or play minigames with friends would be nice and take not as much time to implement. (taking NV as an example. The casino games are already there...)

Co-op might be cool for a new Fallout game, it's far too late for co-op to be implemented for New Vegas.

I heard once that Van-Buren was going to have a co-op system.

If it's an MMO sure, but only if you are aloud to kill however you want, anything else no.