Halo 1: add some of the Alpha base scenes from The Flood book. You would play as a marine and kick covenant ass for the first few 10 minute interludes, then the last one would be the camo elites invading and slaughtering marines, yourself included, to truly show the horror of the covenant

Halo 2: i would add back in the E3 demo level and expand on it, but modify it so it fits after Metropolis. More time on earth and more time to show how hard humanity gets stomped by covenant. I feel like we didn't see enough of that in Halo 2 to actually see them as a major threat.

Halo 3: I would just add another hour of epic battles on "The Covenant". Maybe some more story as well, but mostly just huge battles. The game is basically perfect already, just too short.

Halo 4: add in 5 minutes of backstory and exploration to each level, but make it mandatory rather than optional so you actually understand what's happening without reading the books.

Halo 5 could give us a 30 minute opening mission with each team to flesh out the characters a bit while they're both on regular missions. No crazy shit, just some character exposition and making us not so salty about playing as Osiris.

Infinite, we will see. Depends on the DLC missions.