Halo Reach, with all of its DLC's, 1080p/4k at 60fps.
Bring back the original armor customization systems.(For bonus points, have it so that the game can read, based off of server data, the armors you've unlocked in those games, so you don't have to restart your progression.)
Halo 4 DLC armors and weapon skins.
Greatly expand Halo 2 Anniversary's multiplayer. More maps, more armors, more weapons, more vehicles, more everything. Perhaps they could rename it to Halo: Anniversary multiplayer. Using polls, add fan favorite maps from all Halo games, like Valhalla, The Pit, and so on. Add in weapons from more Halo games, like the spiker, Mauler, gravity hammer, forerunner weapons, needle rifle, plasma repeater, storm rifle, plasma caster, plasma launcher, focus rifle, and so on. Add in the gadgets from H3, and the armor abilities from H:R/H4. Add in things like the rocket warthog, the hornet, the falcon, the Spectre, the chopper, the prowler, and so on. Make it the biggest Halo sandbox to date. Of course, that could, and probably should, be saved for Halo: Infinite, but that's besides the point. I just want more Halo 2 Anniversary stuff.