Defensive turret that hovers in the air.
All the old C&C games, and pretty much all RTS's for that matter, have had base defenses--basically towers that shoot at stuff. The standard counters are using units against which it is ineffective (like tanks against an MG nest) or artillery. I think it would be interesting to have the option to build turrets that can hover. These would be either stationary or very slow moving, and can only be attacked by units that can hit air units. In particular, this means that they're immune to conventional artillery. Being high above the ground, they would have better sight range and possibly longer range weaponry than other turrets, but would be more expensive and/or fragile and/or less damaging, and would use significantly more power. You could also implement a mechanic where if base power goes down, it crashes down to the ground and is reasonably damaged. They also have to be used carefully--since they're not on the ground, they can't be used as a physical barrier to stop fast ground-based raiders from getting into a base--walls or ground units would have to be used for that.
--Provides bases with a counter against artillery
--Attackers have to bring anti-air along
--Anti-air-only units, like air superiority fighters, are now more useful in attacking bases
--One more reason to hit an enemy's power grid
--If somewhat mobile, can provide another option for the turret push strategy
--Has to be balanced so turtling doesn't become overpowered, but not so nerfed that no one would use them