Having played StarCraft 2, this was a really cool feature where you start off with a fairly basic Kerrigan or Nova and then over the course of completing bonus objectives you gain points to unlock additional equipment and abilities for that hero unit until they became pretty godly by the end of it. If you played Heart of the Swarm you know what I am referring to. 
So if they did a Banished campaign they could do the same thing with Atriox, since unlike a Spartan he would have access to all this crazy equipment and would reflect how good he is as well as add a very physical presence to the game.
Plus, if they did add Promethean or Flood faction you have a lot more scope to go down this road. Perhaps you can upgrade or specialize your flood army like you can the Zerg in HoTS. Or, perhaps you can change between a selection of weapons and forms for the Warden Eternal; getting progressively better as the game proceeds.