The only way I feel making a multiplayer campaign could work would be to remove pausing. This would change the nature of the game so that the player would need to reach a safe place before managing his party or planning, but I don't view that as a bad thing.
I also don't see the possibility of having lots and lots of players in the world at the same time. The game would simply be too chaotic and technically demanding for more than 5 to 10 people to populate the game world, if you ask me.
Basically, I think if a multiplayer campaign mode were to be developed, it would be for private games. I don't think a public campaign server would work, really.
I'd go into it more, but I don't think the chances of it being an official feature are great enough to devote more than a few minutes to discuss this right now.