A multiplayer campaign would be amazing. I've got some ideas, but none of the experience needed to implement them.
As stated before, players could take over existing companions. Maybe they could choose a companion, kind of like the custom commander mod.
Players could make their own character like in the regular campaign, with their own background options and stuff. I would rather have this than the first idea, although it'd be great if players could choose between the two.
Maybe the host could be like a regular SP character, but the others could be like the classes in MP. I wouldn't really want this, as it would be kind of boring for the other players.
I could see multiple parties working as long as everyone would be on the same page. For example, Player 1 gets in a battle. If Player 2 is kind of close to Player 1, he/she could join the fight. If they're far away from each other, Player 2 would be forced to spectate the battle. That way, Player 2 wouldn't be able to run around the map and wreak havoc while Player 1 was fighting, and it would only need 1 server (I think).
It would probably be tedious to switch the map whenever somebody would want to get quests, so it might be a good idea to have a lot of stuff be menu based. It wouldn't be nearly as much fun, but it'd be better than nothing. Maybe each part of a castle/town could be made into 1 map (so players wouldn't have to load a new map to go to the tavern, arena, or see the lords).
There are loads of other things to consider.
1. How would saving and loading work? Would the host load a file from his/her computer?
2. Would players join whenever they wanted, or would they have to join some kind of lobby before the host 'started' the game?
3. How would the campaign map even be accessed? Would it have a separate game type? How would it show up in the server list?
4. How would dedicated servers work? Would the game pause until somebody joined?
5. If players were in 1 party, how much control over troops, inventory, and money would each player have?
...and the list goes on. With so much stuff to go through, it's no wonder nobody has tried to make a multiplayer campaign.