I cant believe this didnt occur to me 'til just now. With the introduction of bosses in Warzone such as the Warden, Elder Hunters, Baron, Zealot Assassin, etc.
What if the next iteration of Firefight we had boss waves, ala Gears 3? Imagine it, after ROFLstomping through several waves of Promies/Covies/both, you now have to survive against a barrage of, say, three Wardens along with an army of several Knights, a couple dozen Soldiers and several dozen Crawlers.
This would certainly give me and my friends a new replacement for Horde mode during our game nights.