It could be small things like having hog gunners refrain from taunting enemy corpse when there are still dangerous enemies nearby. If the target is down then immediately move to the next (even if it takes less shots to kill the target than the gunner's preset burst duration).

Or how about ensuring that NPCs get in the vehicles FAST. I would sacrifice beautiful animations for fast boarding time. I don't care if they almost teleport into the seats.

Then there are medium difficulty things...

Then drivers could have some idea of when your shields are down/low and what to do about it.

Gunners better at prioritizing targets.

Then there are hard difficulty things...

Drivers could be good at driving.

Anyway, 343i should take a good long look at all the mechanisms that have accreted/rotted over the years and should make them better. This would involve playing the hell out of small sections of the game with an eye towards improving the player experience. Change stuff, add stuff, refine stuff.

Bungie were a bit slack with the AI in later games. Do you remember Brutes deploying bubble shields and then wandering outside of them as if they weren't there? Do you remember Noble team being "kickass" by moving slowly when under fire? Jun opening fire when he should have been sneaky? Instead of improving the AI to shoulder the story burdens placed on the characters they just made them move slowly when under fire. That wasn't good enough. 343i should do better.