I think they’d retcon the UNSC as bad guys and you play as a resistance fighter fighting for “freedom” as you mercilessly slaughter millions of marines and Spartans with a robotic companion and the other factions of Halo just straight up wouldn’t exist. Maybe the Forerunner’s tech might still be there but the dastardly UNSC would be plotting to use it to kill off the remaining resistance fighters before you stop their plan in their tracks.
I’m using titanfall 2 as inspiration cause they for whatever reason turned the IMC into the evil bad guys and I saw a bunch of people stating Respawn would do a better job at making a Halo game. Which if you saw how their second game turned out I would be laughing.