Every single enemy as of now has been practically similar to each other since CE (besides Prometheans getting an overhaul). What would you add to make each enemy more interesting to fight in Infinite?
For example:
Elites - Elites are Sangheli full of honor. If with a group, they would be themselves. But if once their squad if dead and they last remain, they will switch out an energy sword for a final stand. The only ranks in an elite that this mechanic wouldn’t apply to would be the Minor elites (as they are too lame) and the Zealots (as they will always have energy swords). Keep in mind, the covenant are broken. I thinking whoever holds an energy sword is irrelevant now unless its their rank that permanently holds em.
Jackels - Jackels have always been the same, they were either a sniper or holding thou shield. Maybe they could be both. Depending on where the mission spawns them, if they spawn close, they would have their shield out (And for my case, it would be a retractable shield now). If all power units die near them (such as spawning near elites, hunters, higher rank Jackels, anything but grunts), they would script themselves to a distance and become range with a Carbine. They wouldn’t go as far as sniper range, just as far as a Carbine range, like, mid fight. Any
Sniper jackels already with their beam rifle at their scripted spawn position. But getting too close, like BR close, they will pull our their retractable shield and fire from mid-range with plasma pistol as an average jackel.
Grunts - Lots of Grunts meaning lots of mechanics. Effects near elites non-change. When near a group a Grunts, they gain courage. With a full group of grunts, they become more aware, shift around better, throw more grenades. When few grunts die, some grunts will coward away and their buffs with full grunt squad disappears. When the majority of grunts die, more coward away and some will become suicidal grunts. If near a Grunt Mech, they keep all buffs and no cowards (no matter how many grunts die) until the Grunt mech is destroyed.
Hunters - As known, they can switch their cannon modes and become enraged when one dies. So Hunters are always out for a fight practically. So in order to take them on, you have to keep yourself in the fight. If you need to back away safe from a lone Hunter for any reason, the Hunter will script itself back to his dead brother, share some Lekgolo worms to it and revive its brother. This mechanic could still have the effect on losing their rage on lower difficulty, but still have the effect on higher difficulty (Legendary). This making Hunters more resistant and forcing players to approach them more tactically.
Brutes - Brutes are like Elites but no honor, and more like Warmongers. They have less tactical play and more… brute damage 
! I personally prefer them having destroy-able armor like in Halo 3. It helps with the mechanics I made up for em. If without armor, they will have 3 actions they take. They become rage monsters as you saw in Halo 2, 3, odst, reach. Or they have will run at you at fast pace to ram you (I think instakill on legendary). Or they just end up having a higher rate of fire on their weapons. Keey in mind, there will be those preparation animations for each mechanic that happens, so, you could know what happens next.
Drones - If these suckers ever come back, give em some color than looking like a booger moth. Just allow em to become a simple enemy with ranks. Because we haven’t seen much of em to know what they do.
It doesn’t have to be as detailed as me, but it can be something. I just rather avoid them having resistant buffs and more animated mechanics instead.