What some people are missing when op says multiplayer, he doesnt mean massive multiplayer, like Skyrim conversion to mmo, but the possibility of instead of having a follower for example to play with a friend.
He DID say "at least online PvP", which is ridiculous in a game where you can make a 58 million % Fortify One Handed. While playing with a friend would be fun, playing a game in which there is no definite goal (you can ditch a quest halfway, for example)
may (not always) ruin the experience. S/he wants to go here, you want to go there. S/he gets in your magic, s/he did something you don't want to do right now, etc-etc
It would certainly be better than the Follower, yes, but multiplayer is not the solution for that particular aspect