While it hasn't been confirmed, Gears 4 seems like a good candidate for the framerate doubling enhancement that Microsoft have talked about. I expect it will run at 60fps in Series S and X but we'll have to keep our fingers crossed until MS provides a list.
As for MCC, cross platform play is the biggest missing feature. Then it's time to restore the og Xbox Halo 1 graphical effects and maybe the project comes close to being done. Just bug fixing and ongoing seasons, unless there's something I'm forgetting
Halo Online maps, custom game browser.