Halo: The Master Chief Collection is getting a Xbox One X Enhancement that will overhaul the entire experience. With that update, the group that is handling the Legacy side of things at 343 has been taking in requests. It's been heard loud and clear from their side that Halo Reach and Halo 3: ODST Firefight are being considered. These two aspects are essentially the only part of the series not in the collection at this point. Halo Reach is a prequel to the series going over the events during the fall of Reach and it was Bungie's last release before leaving the franchise.
It's been long desired to be part of this collection as it's also the only notable non-remastered game in the series. Firefight is a staple in the series as a side mode bringing wave based combat, it was a large part of the ODST package and while the core campaign is present in MCC this aspect was not. Both of these are being considered, but neither will be available right away as they're larger in scope. Official statement is here "We’ve heard loud and clear that this is at the top of requests when it comes to new future features for MCC. This is outside the scope of this current update but is something the team will continue to assess and evaluate for future feasibility." Hopefuly it does become something that can be provided, we'll have to wait and see what happens with it.
Read our Halo: MCC Review
View our Halo Hub