Keep in mind this is a rough idea.
However, the point of this idea is making the introduction of Jerome-092 and Red Team in Halo FPS collection, focal point.
I also do not allow anyone to take any of these ideas and try to pawn them off as their own. These are my original thoughts and creative ideas.
DLC starts with Chief finding audio log from UNSC scout team tasked with setting up re-supply drop for Red Team.
In the log, Chief hears scouts screaming about creatures rising out of nowhere from ground, gun shots, grenade explosions, and non-human noises. After period of silence, sound of titanic collision can be heard, running footsteps, and then Jerome-092’s voice comes over as he expresses sympathy for fallen Marines and telling rest of Red Team to follow VISR clues down into Delta Halo to chase the threat - all the while oblivious of the audio log capturing the chatter.
Chief then flips on VISR and sees Red Team footprints leading to edge of Chasm. He stand at the lip for a moment peering over confirming with the weapon that Red Team went down. She confirms and Chief plunges into the darkness.
Game launches quick cutscene showing Red Team landing in darkened, sludge like knee-deep body of water as they continue tracking VISR heat signatures. Game then transitions to Jerome-092 first person as you now play as him for this mission.
Throughout this mission and the DLC, you encounter new enemies capable of appearing from below and above and possess ability to turn their body into blade like objects and can throw “knives” at player. Also, have a “hive” like communication and ability to condense into one singular entity that focuses fire or launching one large projectile every few seconds - called, “The Scourge”
DLC proceeds with switching between Chief and Jerome until they meet about halfway through and Red Team fights alongside Chief. At one point during DLC, Douglas 042 sacrifices himself to allow Red Team and Chief means of escaping.
Throughout DLC, Chief and Red Team are being hunted by spec ops Elites and Jackal snipers all the while fighting the scourge.
Turns out, the Scourge are the result of Flood and Promethean DNA splicing in attempt to use Promethean RNA to control Flood parasitic tendency. Instead, scourge becomes formidable enemy combining Flood ability to consume all bio mass/ease of movement with Promethan combat prowess/armor.
During DLC, it’s learned that the Didact’s Vestige is a weapon capable of summoning Promethean spectres designed to act as sentinels for Delta Halo and self detonating throughout the ring in the event Flood overwhelmed ring defenses.
Banished want the didact’s vestige as they believe that by activating it will rid the ring of all UNSC and human life while failing to realize that it will detonate the ring.