There should be mobile Turret units carried by a small group of Grunts or Marines which would be a hard counter to air units.
The basic infantry are supposed to be strong vs. air units but they really aren't, I mean, it takes a little long for a air unit (other than a Vulture) to kill a group of Marines or Grunts, but the infantry don't do enough damage back. So, I think a good fix to this would be to add a Turret infantry unit, which, just like they are in the Halo fps games, would be strong vs. other infantry, aircraft and scout vehicles, but not heavy vehicles.
I would make these be available after your first reactor or temple and cost 300 supplies and take a little long to build. It would fit in the barracks/hall on the top part of the circle menu and it's upgrades would be on the bottom.
It would be a pretty slow unit, as you would expect, and I think it would be a good idea if it would work like the Cobra in how wherever you press Y it would go there and set up and stay planted there until you press Y again, only with the Turret it can only attack when locked down/set up. And it would only make sense in Halo Wars that the Turret would be able to move a full 360 around. This would make Turrets useful for defending buildings like Forrunner reactors or supply poopers or for fortifying weak spots on you base. Also, if you like to use elephants these used with them I'd imagine would be a pretty strong combo, add a couple locked down Cobras and you got yourself a pretty potent field base. The Turret would be a good early defense vs. hog or leader rushes, one or 2 with a few other units should fend off most rushes.
I'm thinking the Turrets would have 3 upgrades, maybe a speed upgrade called Light Weight Material that decreases how long it takes to pack up, set up and move around. I think a good upgrade for the Covie's Turret would be a little shield bubble that surrounds the Turret, which would work just like any other shielded unit. A Human upgrade could be to upgrade the Turret to a Gauss Turret or Missile Pod for more damage and making it somewhat more useful against heavy vehicles. And another upgrade for both Human and Covies would be to add 2 extra Marines armed with grenade launchers (like the Warthogs) or Grunts armed with fuel rods who, when you press Y to set up, will set up a couple defensive barriers and take cover behind them to give the Turret a little more defense, particularly against vehicles.