Has to be three or four, but three makes more sense.
Dawn
Chief wakes up. Dawn is boarded by a neo-Covies. Neo Covenant fleet outside. The Dawn and some of the Covenant Fleet (all?) are forced inside Requiem.
Requiem
The Dawn lands on Requiem, Chief goes exploring. Cortana establishes the place is Forerunner and possibility active. More skirmishes with neo-Covies.
Forerunner
Infinity is forced to land on Requiem. Chief and Cortana decide head over to it. Encounter with Cryptum and the Promethean units for the first time. Concludes with Cortana and Chief linking up with the Infinity crew, Chief is "cornered" by a massive Promethean strike force in the end but is saved either by some new Cortana power (that ties into rampancy), UNSC, or Chief just badasses his way out, or somehow gets tossed into the new desert environment.
Infinity
Desert vehicle heavy mission with the Mammoth and neo-Covie ground forces.
Reclaimer
Chief is explored as a character. May involve a Library-like level where you're fighting your way through countless hordes of Promethean sentries, fueled by either Cortana or otherwise cryptic messages to fill you in on some new story elements.
Shutdown
Cortana's rampancy subplot potentially peaks here. The Infinity could be not allowed TO LEAVE THIS PLANET on this mission.
Composer
Ancient Evil is revealed to be (or is at least relying on the powers of) The Composer and you're forced to awaken the Didact to help you fight him. Revival plays as Didact awakens.
Midnight
Boss battle followed by a Warthog run. You might somehow end up at one of the other unexplored Halo rings which is now near Charum Hakkor.
Need to figure out where the Halo ring and the destroyed planet fit into this.
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