I have an idea (that I'll be trying to do myself).
You could have your campaign in parts: The story part (maps in the campaign file), and individual multiplayer "events" (such as the siege of an important city, two parties meeting at a crossroad, or a "Big Boss" sort of deally). Granted, the multiplayer maps would have to avoid having the main heroes as much as possible for continuity (unless they have a specific reason to be there).
Other than that, I really don't know how you could do a multiplayer campaign aside from what DioD suggested. |