TL:DR: What if Didact becomes the intelligence behind a new sect of Flood. One set on destroying humanity rather than testing them, and then wants to use the biomass from a infested humanity to finish off his fight against the true flood. See below how I envisioned this scenario to play out.
I had a dream where the absolute record was flood infested and Jul cleansed the immediate area of flood presence and inserted a glowing cube that held Didact’s consciousness into a console.
The console was attached to the flood intelligence and they tried to absorb him into the collective consciousness, but his mind was too fragmented, and his only desire was to destroy humanity, which was incompatible with the Flood’s desire to test humanity.
So he hijacks a large swath of biomass, uses his prometheans to subdue Jul’s forces and converts them to flood. His flood.
Jul and Halsey manage to escape with the Janus Key and are left drifting in space when Didact self destructs some crawlers in their engine room. He cannot have them interfering with his plan anymore.
He begins boarding their ship with wave after wave of prometheans and His Flood and Jul’s forces fight valiantly to protect Halsey and the Janus key but begin to lose.
A lone prototype UNSC fighter craft appears out of slip space. Blue team has arrived. They board the ship with lethal precision and extract Jul and Halsey.
John leaves Jul’s ship on self destruct and it blows right as they escape, severely limiting Didact’s options as his newly claimed biomass is cut by half.
Our heroes make it to the absolute record, and insert the Key back into the console. Screens glitch and lights dim, and a hologram appears.
A fragment of Cortana who had been in Didact’s body armor when he was composed spoke.
“John, I told you I would always look out for you. There isn’t much time. Didact intends on taking Sanghelios, infesting the Sangheli, making a flood army. I’ve heard his thoughts and lived his madness.” He intends to use humanity’s greatest foe as his own.
There is a way to stop him…The Guardians. When the Forerunner’s rose up against their creators, they hijacked their creators greatest weapons. The Guardians were the peacekeepers of the Galaxy. Protected all of creation from calamity, each other, and from extra galactic threats. This was revealed to Didact in a conversation eons ago with…
John - Cortana!!! Cortana!!!
Gravemind - We meet again warrior. What’s left of your construct is mine now. I can’t let her give you all of the pieces of the puzzle. For now I leave you in contemplation. Know this, warrior. The age of retribution is at hand.
(Absolute record shakes as a massive flood ship leaves the area.)
Jul vowes to make it to Sanghelios to prevent Didact’s plan
Halsey reveals that she knows how to activate the Guardians, she needs a shard of the crystal found on Reach so many years ago. Jul says the crystal shards were recovered by Sangheli at the end of the Great Schism and reside on Sanghelios. Knowing there is no time to waste Halsey uses the conduit and sends Jul, his remaining warriors and Blue team to Sanghelios, to recover the shards and to prevent the threat of Didact and His Flood.
Meanwhile, Spartan Locke is on Sanghelios investigating the location of the Master Chief. Waiting to meet with the Arbitor.