I'm normally with the NO crown on multiplayer, but wouldn't it be kind of cool if a friend could take over as one of your companions like in Fable II. VATS would be pretty confusing, but it still might be useful and fun.
I remember Saints Row 2 allowed players to pause the game during co-op, but I think you were also able to unpause them. Also, I believe the owner of the file the co-op was going on in was able to kick the other player if he was getting annoying. If it was implemented well (which is hard to be honest), then I wouldn't mind co-op with another player taking place of a companion or custom character that you can give equipment to. Also, I would try to find somebody that acts like they are in the wasteland with me. However, Fallout is most likely not the game for this. One thing that would be cool is if your stats are recorded online. For one example, this would be able to show your friends how many hours you wasted playing the game.
Spoiler Those aren't hours wasted.