Each unit would have various available upgrades to weapons and armor that would modify their combat roles as well as multiple autocasted abilities.
(note: this is a quick sketch of how I would love to see the units/tiers organized, names and abilities can be swapped around and changed)
INFANTRY:
Tier 1
Covenant- Grunt Squad (light cheap infantry with plasma pistols and grenades, can upgrade to needlers or fuel rods and various armor upgrades)
UNSC- Marine Squad (light infantry with battle rifles and frag grenades, can be armed with assault rifles, rocket launchers, and various support gear)
Flood- Polyp Swarm (A large swarm of flood infection forms, suicide unit that can also be used to heal other forms or turn slain enemy units into combat forms)
Tier 2
Covenant- Jackal Team (light mobile infantry with carbines, can be upgraded with sniper rifles or shield gauntlets and receive skirmisher armor)
UNSC- Assault Engineer Team (support infantry with shotguns, can build cover emplacements and heavy weapon turrets and be upgraded with flamethrowers and cyclops armor)
Flood- Stalker Form (stealth assassin, close combat highly mobile flood form with leap attack. low health.)
Tier 3
Covenant- Elite/Brute Pack (advanced short and mid ranged infantry, can be upgraded with advanced armors and mobility upgrades as well as heavy weapons)
UNSC- ODST trooper Squad (advanced mobile infantry equipped with short and long ranged weapons, various armor and jetpack upgrades)
Flood- Ranged Form (Mobile ranged flood form, high anti infantry damage output high mobility)
Tier 4
Covenant- Hunters (heavy armored elite infantry, heavy anti vehicle weapon high armor high health low mobility)
UNSC- Spartan (elite heavy infantry, high mobility, anti vehicle abilities, advanced weaponry, very long training time)
Flood- Tank Form (armored charging infantry, regenerates health, emits flood spores)
VEHICLES:
Tier 1
Covenant- Ghost/Chopper (fast anti infantry scout vehicle, limited upgrades)
UNSC- Warthog (fast scout vehicle, various armament upgrades)
Flood- Stealth Form (fast scout form, low damage difficult to detect)
Tier 2
Covenant- Wraith Tank (medium mortar platform, can be upgraded with anti air flak weapons and armor upgrades)
UNSC- Mantis walker (medium mobile rocket platform, can be upgraded with various weapons, limited armor)
Flood- Infector Form (medium infection form spawning flood unit)
Tier 3
Covenant- Locust Walker (laser artillery platform, various armor upgrades)
UNSC- Scorpion Tank (armored tank, various armament upgrades)
Flood- Juggernaut Form (large deadly flood form, high damage close combat attacks)
Tier 4
Covenant- Scarab Walker (super heavy mobile weapons platform, various armament upgrades)
UNSC- Elephant/Mammoth Heavy Support (heavy mobile base, various support weapon upgrades including long range missiles)
Flood- Devourer Form (large slow high health flood form, grabs units and pulls them, produces spores and combat forms)
AIR:
Tier 1
Covenant- Banshee (fast anti armor fight/gunship)
UNSC- Hornet/Hawk (light interceptor/gunship)
Flood- Swarm Flyer (light interceptor/gunship
Tier 2
Covenant- Specter Heavy Gunship (troop transport/gunship)
UNSC- Vulture Gunship (troop transport/gunship)
Flood- Abductor Form (troop transport/gunship)
Tier 3
Covenant- Vampire Fighter (air superiority fighter)
UNSC- Shortsword Bomber (long range fighter/bomber)
Flood- Bomber Form (slow spore pod bomber)
Tier 4
Covenant- Lich Heavy Support (super heavy group support)
UNSC- Vulture Heavy Gunship (ground bombardment and missile support gunship)
Flood- Greater Polyp Form (high health fast air superiority flood form)
ORBITAL
Tier 1
Covenant- SDV Heavy Corvette (anti air fast attack ship)
UNSC- Fast Attack Corvette (anti air fast attack ship)
Flood- Flood Spore (cheap orbital suicide unit)
Tier 2
Covenant- CAR Frigate (fast light armored support craft)
UNSC- Stalwart Light Frigate (fast light armored support craft)
Flood- Flood Meteor (fast light armored support craft)
Tier 3
Covenant- CPV Destroyer (heavy armored orbital combat ship)
UNSC- Halberd Destroyer (heavy armored orbital combat ship)
Flood- Flood Asteroid (heavy armored orbital combat ship)
Tier 4
Covenant- CCS Cruiser (heavy support bombardment ship)
UNSC- Haclyon Cruiser (heavy support bombardment ship)
Flood- Flood Mass (heavy support bombardment ship)