I think there should be a co-op feature where you can instantly join a friends game, without interfering the persons game. So when you join a friend you are play as your character that you have in your single player campaign and go around with your friends leveling up and completing quests together. Then the things you earn in the co-op game carry over to your single player. When you decide to leave your friends game it doesnt affect him and he can just carry on playing.

Maybe join http://www.gamesas.com/index.php?/topic/1154646-official-discussion-of-multiplayerco-op-in-skyrim discussion?