If anyone has ever played Dawn of War: Dark Crusade they added a tile based campaign where you could play as any faction. It worked by using the multiplayer maps, mixed with a more set piece stronghold and these had more story elements attached to them. So basically you could do something similar in Halo Wars 2 where you play against the AI, or a human player like in Total War, and fight to conquer the Ark. I don’t think that would take too much work since you basically just be fighting a series of multiplayer battles but with a UI to map your progress and you could maybe interweave more set piece campaign style missions. We know we are getting a 2 mission campaign expansion soon. Well, in the future 343 could pad that out by inserting those large scale set pieces into a wider map based battle. You could then add stuff like power ups and map specific boosts to add variety.
I just think stuff like this in Total War, Dawn of War and original Battlefront was a very clever thing to do. It felt more involved than just playing a few matches against the AI and has a little more player choice than the completely set piece linear campaign.