ADD:
1. The Minor
Armor customization (preferably more than what Reach had) and armor color: Primary, Secondary AND armor detail
Bright, Very visual lighting like in Halo 3
Equipment, they were much more balanced than armor abilities
Halo 3 grenades, the ones in Reach were way too powerful
New vehicles, a flyable Pelican or other new vehicles would be cool
Some of the old Halo 3 Maps like Guardian, The Pitt, and Valhalla
2. The Major
More customs games like in Halo 3, customs were what kept Halo 3 great for such a long time
Forge, bring back the forge heavens that were Foundry and Sandbox, and I would like to see decorative objects like in H3 and not just structural
Firefight, gameplay and maps like in ODST, custom options like in Reach
IMPORTANT: The music chanting in Halo 2, H2 music captivating and perfectly conveyed the epic struggle over Halo. I remember the first Halo I ever played (Halo 2) At the main menu listening this track play () I didn't even need to play, I knew this was going to be something great
Multiplayer just like Halo 3, not the randomness of Reach
DON'T ADD:
Any armor abilities EXCEPT sprint
Reticle bloom
The DMR (the new BR is all we need)
Health