Multiplayer:
More customization, unlockable camos, not just from specializations.
You should be able to change colors of the plasma on covenant weapons and the light details from forunner weapons
More unique loadout weapons
Ordinances were a great addition, but They could expand on it alot.
You could have ordinances that drop airstrikes and vehicles in big Team and custom games
Which leads to...
Forge/custom games:
Bigger, open forge landscapes, i miss forge world... but i dont Miss playing on The same map every time
More vehicles, and bring back the falcon! And it would be sick if you could just be a passenger in one, like a warthog, and shoot with your gun.
I also like the idea of firefight 3.0, where you can unlock new areas and guns and a pelican could fly you around to new levels and areas. Just going around on a customizable adventure with your friends in a huge landscape gives me the tingles in silly places.