For those who want a "multiplayer campaign" in the sense of a living, breathing world with hundreds of other players, is the gold standard.

The Ultimate Guide to Mount & Blade: Warband Multiplayer Campaign Mods (2021 Edition)

One player hosts a standard single-player campaign. When they enter a battle, they can host it as a multiplayer match. Friends can then join the "server," take control of companions or specific troops in the army, and fight the battle together.