Back in the day, if you were around during the early days like myself, you will probably remember a limited time gamemode called Versus Firefight (or Firefight Versus, either one) where two players would spawn as Elites and two players spawn as Spartans.
I was thinking about it this morning, and there could possibly be a rather unique opportunity of this mode returning in the form of Flood Firefight, which is set to be released in the coming months.
Game difficulty is set to Heroic, and all four players start out as ODST’s, pretty much the same as Heroic Firefight. When a player is killed, however, they don’t just sit idly by, waiting to respawn; they instead join the Infected and have to try and kill the other remaining players. Granted, a new character model of an infected ODST may have to be created, but I think it would be a rather interesting gamemode.