With the constant requests of Halo 3: ODST’s Firefight mode to be added to Halo: The Master Chief Collection, it is very clear that Firefight is a fan-favourite game mode. It was first added into the Halo games in 2009 and then brought back in 2010 and then 2015/2016 with Warzone Firefight but you could only fight Covenant and Prometheans (not counting Terminus Firefight as that is in a spin-off game).
If Firefight was to return in Halo: Infinite, this is 343i’s chance to shake things up. To add new factions for us to fight against: Banished and, more importantly, the Flood. It doesn’t have to be canon but just to add some variety to the game mode.