Overall Halo 4 was a good game but there were some disappointing aspects about the game that should be changed:
1. Avadible species to play as
Very many people enjoyed being able to as the elites in Halo: Reach while also having to option to play as a spartan. This allowed for the player to experiance the combat styles of both the spartans and elites while also having special assassinations tailored to both of them.
2. More team premoting game types
The gametypes of dominion and oddball were great in Halo 4 because it was easier to win as the people worked in a team giving them bonuses to help them win. So I think it would be benificial to add more team elements back into the game like the popular gametype from Halo: Reach invasion. Though if you do bring invasion back add an animation at the beggining and end of the match to show the deployment of the attackers, and the retreatment (attackers lost)/ escape (attacker won) to give the match a better feel. Meanwhile give invasion a more sensable feel as the spartans attack covanet or promeathean locations (covanent cruzier) while the elites attack spartan locations ( the infinity per say).
3. Assassanations
The assassanations in the Halo 4 multiplayer were amazing no complaints here, but in the spartan ops and campagin seemed to be lacking. I feel the should have been more special assassanations with the energy sword. Also you could add a special assassanation on the hunter like from halo forward unto dawn, but the player has to have grenades.
4. Vehicles
Good addition to the vehicles in Halo 4 and graphic changes to them, however the loss of flying vehicles for the humans severly hurt Halo 4's multiplayer community and forge. You may also consider adding a promethean vehicle that is a mix between the ghost and warthog.