In H2, there were essentially NO boundaries in Campaign, which added hours of fun exploration out of the map.
I also think that crazier skulls would make Campaign funner. One that increases your speed to like 200, one that increases your jump height, one that turns all enemies into rag dolls when they’re hit with a powerful force, ect.
And of course, Easter Eggs. The Scarab Gun was the best example. We need stuff like that.