The biggest thing you can do for MCC, is give it more spotlight, I get Halo 5 is the newest and shiniest toy that you have to show off, but think about how big Halo was with Halo 2 and 3. It was one of the biggest eSports for it's time, get rid of Halo 5, and go back to 2 or 3 and watch the community instantly grow overnight. If you anyone has doubts on why anyone would ever want to go back to old game, I'll throw a beautiful example at you.
Super Smash Bros. Melee.
This game came out 17 years ago, and is one of the biggest competitive fighters out there, getting thousands of people entering it at every major tournament, Halo 3 doesn't even look that bad, and has aged fairly well, or look at remastering it, the same way Halo 2 was done.
Some people may well the Halo 2 re-master wasn't great because the map pool was lacking a bit. Halo 3 had an amazing selection of competitive maps, and the forge maps that were implemented by MLG such as Onslaught was great.
I love this series, and hate seeing it do so poorly, when it used to be a juggernaut before poor design choices towards multiplayer.