Gameplay
1: Add evade to the controls. Halo 4 added the sprint function already and thruster pack is almost useless compared to the other abilities. They could have you hold a to evade, and press it for the regular jump.
2: add meters to the sprint and (if it's in) evade. Makes it easier to tell if you can still use them.
3: Bring back playable Elites. They might be able to have them in Spartan ops. Since the storm covenant elites are only a part of the total Elites.
4: Bring back duel-wielding. I would also like it if you could even holster duel wielded weapons without dropping the second.(provided they can do this w/o it being OP, I think they could pull it off though) Maybe add to the firepower support package by allowing you to start with duel secondary weapons.
5: I know I'm beating a dead horse, but bring back fire fight. And expand it to include forge and optional boss encounter such as scarabs, war Sphinxs, and Drinols(Go down to the "Enemies" section)
Vehicles
1: Add the locust and vampire from halo wars
2: Make most vehicles faster. This will hopefully keep splattering viable if evading is put in.
3: Add the spectres from halo 2
4: Add vehicle-to-vehicle hijacking. I think it would be cool and open up more tactical options in vehicle-on-vehicle combat. If it's too OP, they could limit it to only doable by passengers (for e.g.: a guy riding shotgun hijacking an enemy ghost alongside him)
Enemies
1: For hunters I want two different types: one that fires its cannon like the fuel rod cannon like the ones in halo 1 & 4, and the ones that fire beams like the halo 2 & 3 ones. They can pair the two types and have the beam ones keep you in cover via suppression-fire while the cannon one fires volleys over your cover as a "cover buster". This can make hunter encounters the challenge they were meant to be.
2: (sort of) New enemy types: The Drinol. For those not in the know, the drinol was a cut enemy from halo 2 that was basically an enormous beast that would charge and flail around and even be capable of throwing vehicles! I think these would be awesome enemies to fight as sort of boss encounters. Also the Forunner war sphinx. for the same reasons as the Drinol
3: Returning enemies: Skirmishers (Halo: Reach), Huragok/Engineers (Halo: ODST, These can also be allies as well since they're technically neutral in the conflict) Scarabs, do I even have to explain why?
Story
1: Show MC's face already! It's been 4 games so far, 7 counting the secondary games, I think its about time they showed it. They've already had his face described (Reddish hair, some freckles, basically ginger)
2: Make the story longer, or make more than 1 campaign. I was kind of disappointed at how short 4 was. I would like a longer story. Maybe have more than 1 campaign. For example: I was maybe thinking they could have 1 campaign with MC and another spartan (Linda-058 maybe? Was she confirmed dead or was that ambiguous?), one with arbiter and another elite, and one with the ODST team from halo: ODST, promoted to spartan 4s between halos 3 & 4. The reason MC & Arby have another person with them is so that you could have a co-op campaign without player 2 being just some random schmuck doing who-knows-what off-screen during cutscenes.
EDIT 3: Keep cortana dead, bringing her back is just going to ruin the impact her death had in halo 4. Focus instead on how MC is dealing with her death.