I agree that Multiplayer questing would kind of ruin the experience not to mention all the stuff they would have to get rid of to mak it fit. However I do think it would be cool if you could load your charicter into some online market/arena area where you could trade/buy/sell different items with others. Also it would be cool to put your hero to the test against other players. I think it would be cool just to see everyone's different charicters too.
I think people would just arrange to give each other awesome weapons and armours, and that would ruin the experience of exploring yourself.....IMO