Covenant:
Soldiers:
Grunt Squad
Grunt Gunners (2 man team that deploys a turret)
Jackal Squad
Skirmisher Squad
Jackal Snipers (2 alien team)
Elite Squad (5 elites; 1 Zealot)
Elite Honorguard
Brute Squad
Hunters
Vehicles:
Ghost (maybe a 2 unit squad to make up for weakness)
Chopper
Spectre/Prowler
Revenant
Shadow (can transport units)
Wraith
AA Wraith
Locust
Scarab
Air:
Engineer
Banshee
Vampire
Phantom (used as a gunship)
Spirit (for transport ability)
For both the UNSC and Covenant, certain units could be condensed into one through upgrades. Another idea I had is that there could be different branches of upgrades. For example you could research the turretless Warthog, and from there research an upgrade, like a Chaingun or Gauss Cannon, and then have access to it on your unit menus.
Leaders and Abilities:
I was thinking that instead of having Leaders with specific units and abilities, the player could just be a generic leader among the UNSC/Covenant, for multiplayer at least.
Once you build a research building, you could choose to research only one of each available ability types (aka MAC cannon). This would allow the player the ability to customize their attack plan.
Offensive Abilities
MAC Blast
Glassing Beam (Covie equivilent to MAC)
Carpet Bomb
Orbital Bombardment (a midpoint between MAC and Carpet Bomb; think a cluster of explosive shots, like Reach’s target locator)
Defensive Abilities:
Cryobomb (will never outright damage units, just completely stops them)
Cover (when activated, all units will position themselves behind barriers, giving them added protection)
Healing
Disruption Bomb (confuses enemies, causes vehicles to slow down)
By allowing any player to mix and match any Offensive Ability with a Defensive Ability, each matchmaking opponent would offer a somewhat unique opposition. Plus this allows well thought out teams of players to make a good all around set up. It would also offer a new balance that Halo Wars didn’t have. For example you could research the Cryobomb and pair it with any Offensive Ability to stop an enemy and damage him, but then you’ll be lacking the Healing ability when you may need it.