• Prometheans
• Armor abilities (Chief's suit broken and he goes to paces were there aren't crates of AAs lying around - like most of the universe)
• Each race having its own, me too, weapon set copies

Don't have the Chief saving the galaxy - he's already done that and it's status quo for him. He needs to take on ONI and the less savory aspects of the UNSC.

Oh and it turns out that the Didact actually died at the end of Halo 4 - a surprise twist that confounds all the narrative norms for this genre. (Come on 343 - defy convention.)

The Didact's death undermines the Storm's morale and the Chief joins forces with the Arbiter to assault the Storm's home world. I see a reenactment of the Ark intro with lines of Scarabs stepping off from an assault carrier's main hanger and drifting down to the Storm homeworld - fiery reentry, concussive landing and immediate deployment into a pitched battle with Storm forces. Waves of Elites landing via drop-capsules.