1. Playable Sangheili (Elites) in multiplayer in every gametype in both custom games and matchmaking like in Halo 3 without restriction (meaning, not Reach style). Canonically this would make sense now since they are working together against various common enemies (it already made sense in Halo 5). In some cases, according to various sources, Sangheili were invited to train with Spartan IV's in wargames. There is no reason not to add them at this point. I could literally care less if the hitbox is a bit bigger than a Spartan, I can deal with it. We already did in Halo 3. If restrictions are necessary for things like ranked, then simply enforce player models for those modes. Bungie achieved this in Reach, and only ranked players actually cared. Social matchmaking, unrestricted.
2. Campaign with multiple plots going (much like Halo 2 and 5 though probably too late for this wish if the Campaign is already ironed out). This could be easily done between Locke, Master Chief, and the Arbiter doing three or more different things
3. Racing like in Halo 3 matchmaking! I had a lot of fun with this.
4. Reach levels of armor customization for both Spartans AND Sangheili (updates these days make it a simple process to add more over time).
5. Covenant Shadow vehicle as a troop transport for forge (it's like a smaller version of the Elephant! https://www.halopedia.org/Type-29_Shadow). Elephant if it's not already planned as well. I have many ideas for these in forge mode!
7. Flood Survival or Promethean Survival (much like horde mode in many other games).
8. If not flood survival, zombie survival much like Call of Duty Nazi Zombies. I think this would be a unique spin on Halo as a fun arcade gametype. Doesn't necessarily need to be in matchmaking but it can be.
9. Spartan Ops that allow both Sangheili and Spartan joint operations. Canonically this would even make sense, as they did this in various missions after the fall of the Covenant already.
11. Ability to have no weapons as a selectable option for a game mode.
12. Large scale BTB (12 - 20 player teams).
13. Spawnable A.I. for forge (and ways to create reoccurring spawns).
The game is more than likely very ironed out by now in development, but I would love to see modding capabilities for client side appearances (much like Left 4 Dead 2 does) for custom games. Since it is coming to PC, I don't see why this wouldn't be a feature down the line. Matchmaking, obviously no modding of any kind and a check should exist that will forcefully remove mods if they are applied.