. Get rid of the triple gun Sandbox that currently houses the BR, DMR and Carbine. The DMR has no place in games that are chronologically past Reach, which Halo 1, 2, 3 and 4 are (As reach was a prologue).
2. Keep the BR as a 4 shot like Halo 2 and 3 and the recent Weapon tuning update.
3. Scrap this leveling system they have now and revert back to the Ranking system in Halo 3; You gain "skill" in Ranked Game modes and do not in casual game modes.
4. Promethean version of the Energy Sword and Rocket Launcher.
5. Symmetrical map design like Halo 3. While Halo 4 had amazing graphics on the maps, they played out terribly and lacked the fludity of Halo 3's maps like The Pit, Narrows, Guardian etc.
6. Make at least 5-6 map remakes from Halo 3 and Halo 2. Preferably all the maps MLG picked up like the Pit, Contruct, Narrows and Guardian. Base the map design on the symmetry of these maps and only delve from it when a game mode entails it; like the bigger maps needed for the bigger team games.
7. In order to get back on to MLG, REMOVE ORDNANCE AND THE CONTRACT WITH VIRGIN GAMING. Halo isn't the same without MLG and it's saddening. Were it not for Halo, MLG would be absolutely nothing. It sickens me to see COD on the circuit and not Halo.
8. With the removal of Ordnance, lessen the amount of power weapons. Only power weapons needed: Sniper Rifle, Rockets, Shotgun (If you count that) and THE SAW (I liked it from Halo 4).
9. On the note of power weapons, have them be at pre-determined locations with timed spawns like in Halo 3. This made for less randomness and rewarded people who were good at the game and knew it well.
10. Make armor abilities less centralizing and instead of being from a load out, make them pick ups and limited on the maps. Camo was perfect in Halo 3 as a power-up because it didn't completely fade when firing like the AA, and it had a longer duration of use, whereas the drawback was it needed to spawn and it was a pick up power-up.
11. On the note of Loadouts... REMOVE THEM COMPLETELY. It works for COD, it does not for Halo. It cheapens the experience and it makes it look like Halo, which promptly revolutionized the FPS genre when it came out, is taking ideas from the most bland, overhyped and overexaggerated shooter in history!
All in all, a mixture of Halo 4's graphics and Armor customization, Halo Reach's Forge abilities (Because of Forge World) and Halo 3's overall gameplay and competitive viability.