If there is any sleight chance to still take this list into consideration;
1. Definitely a must is the Needle Rifle, and Armor Lock above anything else I have listed, personally I prefer them to the Carbine and Shield respectively though i wouldn't mind if the carbine too much if it also is found throughout the game.
2. Don't bring back Cortana instead show or bring in the remaining Spartan-II's and Spartan-III's and of course the Arbiter
3. The Revenant and possibly the Brute Chopper?
4. The Grenade Launcher and The Plasma Launcher
5. The Missile Pod and maybe the Brute Shot?
6. The Plasma Repeater, or just improve the Storm Rifle
7. Dual wield and The SMG
8. Some Forerunner vehicles
9. in co-op campaign do like in Halo: Reach where Noble six's armor appeared as the custom design of the player's spartan armor in matchmaking.