More subspecies and versions need to appear.
So with elites they have different skin types (smooth and scaly), colours (grey, black and brown) and jaw structures (H4/5 and previous) so we should see a mix of them. Additionally armour should vary more and rely on colour to define rank so imagine seeing an elite general with Minor armour or a spec ops with a mix of storm and classic armour. This should apply for players level of customisation as well.
Grunts now have the bulkier subspecies in H4 so again both should return with mixed armour. In a war that's after a war that's after another war you take what you can get. Maybe even more types because grunts breed very rapidly so genetic variation is going to be very high.
Kig-yar already have three types, but let's step it up one more because males and females take part in Combat as far as I'm aware, males have spines/feathers on the back of their necks while females have armour plates because the men of their species get a bit... bitey during courtship. So let's add that into the variety of appearances. Kig-yar are also very ambidextrous so seeing both left and right handed fighters would be a nifty little detail. The slimmer classic Jackal should play defensive roles for high ranked allies (surrounding elites to protect them) where the bulkier H4 version should demonstrate more aggression. Also let them drive vehicles because the can in lore. Mix things up.
Hunters need to have more cannon types. There's currently 3, a scarab beam, fuel rod cannon and now a burst fire shot. Since elites and hunters are buddies and elites get blamite (needler ammo) from a moon of their home world then maybe hunters could have a needler based weapon. Could also have plasma based variant like a walking shade turret. Grenade launcher? Why not right?
Brutes have a lot of body fur but in H3 they shaved for some reason. I think it was something to do with Tartarus' death but idk can't remember. What I'm trying to say here is they should have different levels of facial hair from shaved, trimmed and we'll we know from Halo wars 2 that they can have some cool beard styles. Armour wise they should have a mix as well unless it's the banished because they do have their own set of armour. It wouldn't surprise me though to see members making use of old covenant armour and tech.
Yanme'e were common on brute ships because brutes weren't allowed Huragoks due to the Elites. Heck the elites even deactivated the Grav lifts in many brute controlled ships (prior to the Great Schism) forcing them to rely on ladders. Anyway Drones provided a new airial enemy that was easy to kill but literally swarmed you and the body explosion in reach on headshot was oh so satisfying. ODST added a new rank to drones that I think had a shield and used a plasma rifle instead of needlers or plasma pistols. Seeing them duel wield would make them a little more ferocious to fight.