With the announcement of new leaders as DLC for Halo Wars 2, the first name that pops to mind is Jul 'Mdama. Though not affiliated with The Banished in any way, multiplayer isn’t cannon in Halo Wars so it would make sense to add other post Covenant-war leaders like 'Mdama. Other reasons for having Jul as a leader would be because of his lieutenant, Gek 'Lhar, being able to kill Spartans, it would only make sense to make him a hero unit, his weapon an Energy sword, and his leader ability Active Camouflage. It just seems like a really good opportunity to bring both these great characters back into light, without breaking cannon. Additionally with Jul’s faction being separate from that of The Banished, buildings and vehicle aesthetics could change to look more like the traditional Covenant look, which I am sure many fans (myself included) would be very appreciative of. For unique units, 'Mdama could perhaps have Zealots, that would act very similar to Honor Guards from the original Halo Wars (This is just from the top of my head, I’m sure other could think of better unique units). Finally for the UNSC counterpart in the DLC, they could add Lasky as a leader, with Palmer as a Hero unit, and S-IV’s as unique units.