There was no multiplayer campaign in Warband, so I don't think there's much chance of them implementing it now. Although, there were some interesting mods in Warband, which allowed for multiplayer campaign, so there might be a few mods for bannelord, which allow for multiplayer campaign, but they would most likely have many bugs and glitches, and not allow for a full campaign mode. I could be wrong, it depends on what the developer community comes up with.