I'd say yes to the multi player if it was going to be done right.
No jumping into someone else's games and leaving when you feel like, the world would have to exist when both players are there only and it should not be possible to advanced without the other person present.
It shouldn't be co-op, each play should be able to follow their own non-linear path, and whatever conflicts come from that could cause a really quick end to one of the players ^_^ or they could work together if they so choose.
one person gets to the top of the fighters guild, then just lets the other person take the position under him, that's fine, but you know if the npc's don't think he's earned it, there could be an uprising