Many gunships have been powerful in the past so I don't think air needs a radical change to bring this about.
In general, I don't want Starcraft style air in Zero-K. Planes and gunships should fill a distinct strategic role in the game, and not just feel like floating ground units. More specifically, I'm talking about the high tech mainlineair units of Starcraft, rather than transports and raiders such as Muta and Wraith. Transports and raiders seem to have a unique role on the game. For other units, like Broodlord and Void Ray, the ability to fly essentially does the following:
Confers all-terrain pathing and the ability to (mostly) phase through units.
Changes which weapons are allowed to shoot at it.
Makes the unit use flying armour/weapon upgrades.
We already have all-terrain units of average speed (spiders) so I don't think we need this with the addition of floating. Also, all-terrain is less important in ZK as the maps are much more open, so a flying unit of average speed would behave a lot like just another ground unit.
The design of Zero-K is firmly against weapon and armour types that modify how much damage is dealt. We're also wary of target restrictions as they are basically just 100% effective armour (with unit AI that doesn't waste its time shooting). So any target restriction had better be unambiguous and well justified. The upshot is, if air units start looking too much like floating ground units, then it becomes hard to justify AA as a class of target restrictions. AA would morph from being a nuke/anti-nuke style strategic-zoning interaction into more of a tactical "weapon X is not allowed to shoot unit Y" interaction. Air isn't supposed to fight AA fairly, or even be part of an army that fights AA. Sometimes a big blob of AA is like a weak anti-nuke. To use air in the area you've got to use other forces to destroy it.
Basically, I'm not looking to promote this:
Similar ranges between anti-ground and anti-air weapon also promotes a lot of air-ground combined arms forces in which air hovers over ground. Compositions like Mutalisk Zerging, Marine Science Vessal, VoldRay-Ground, Raven-Mech and so on.
Compositions like this are what I'm calling tactical uses of air units. These are armies of units with somewhat similar speed that move around in a blob and fight. The air units contribute the same sorts of things as ground units: firepower, abilities, and being in a target category. I imagine Muta-Zergling is more like the air/ground combination that I'm going for as this seems like a raiding army rather than a fighting army. But, in particular, Voidray-Ground just seems like a blob that strengthens the army it's in by adding another target restriction class.
ZK air is not necessarily meant to be used in an army blob. It is better suited to rapid response, support, to flank, or to snipe valuable targets. It creates a softer version of the nuke/antinuke interaction to give ground forces unarmed targets of value. Air units are fast but don't have the stats to rival ground units. I'm not opposed to people putting gunships in armies. Nimbus works fairly well behind a lot of armies, and Harpy has in the past. But I wouldn't bring their speeds down to ground units of a similar class.
I'm not sure if I'm even looking to promote meaningful micro between air and AA, or more than exists. The skill test for the player making the AA is a combination of valuation of what to defend and a bit of 'tower defense'. The skill of air is to judge what they can kill for what sorts of losses, and to decide whether its worth it. (Also, You can set fire state to a hotkey in the hotkeys menu.)
The common design in Starcraft is that AA has less range than the longest ranged ground units at play, and this led to a lot of air transport based game play that is one of the coolest part of that game.
On transports, I think the Starcraft transports are unique to Starcraft. They are precluded by various things deep in the design of Zero-K. I'm talking about how fast they can pick up and put down units. This seems vital to many transport tricks in Starcraft. The issue is, in ZK, we try to avoid mechanics that would be ridiculous in the hands of someone with infinite micro, and those that would simply be removed. For example, the spy in Red Alert 2 doesn't have a camouflage ability against someone who can check each of their units every game frame. Instant transports that are tanky enough to exist in battle would warp the game significantly with good unit AI. I'm sure Starcraft gets a lot out of it, but we have plenty else going on.
Less tricky stuff, such as drops, exist in ZK. I've even seen Lance and Crab repositioned via transport. Perhaps they could be a bit faster to pick up and put down their target, but I wouldn't do anything extreme. Also, don't forget that ZK battles already have plenty of tactical mobility as most units move while firing.
There is some nice ideas on how to get fast airplanes and helicopters into the same game without breaking stuff though. Different AA ranges for airplanes and helicopters is one thing, the other is different vision system of air spotting and land spotting, so one can spot airplanes far far ahead of ground forces and thus react in time.
In a similar vein to what I wrote about target restrictions, we value simplicity too much to do this. Weapons should have a single range. Units should have a single sight range. There actually already is a separate vision system for air though - detection of air units ignores terrain. The default behaviour of Spring has units detect aircraft at at 50% greater range. We removed this intentionally.
I have buffed gunships in the past by reducing AA damage and plane health by the same proportion. I think I've done it twice so far, and I'm not sure how much room there is left though. Flex AA puts a limit on how low plane health can go, and the aircraft that are the most resistent to flex AA (Nimbus and Likho) seem to be in a decent spot. So perhaps the answer is simply to give individual buffs to units that need it.
In a infinite eco game like OTA successors, it is only pretty acceptable to have nearly "invulnerable" units like backline map ranging artillery or superweapons as long as damage inflicted is low and barrier to entry is high. As such, all air can be balanced by nerfing attack power.
Many of the nerfs for Likho have been to its reload time. This is a form of reducing its overall attrition power. I'm not keen on reducing its damage though, as that might ruin the feel. Reducing range or speed sound fine though. I'm just not actually sure that air, as an entire category, is too prominent at the moment. So a nerf to Likho would need some sort of nerf to AA, or buff to other bombers, which makes things a lot trickier.
The design space for air is large and not fully explored in zk imo. High alpha bomber airplanes is just one conceptualization of air units....
Yes, there is more to explore in air-space. I recall there being two slots in the build list that anyone could try to fill. Arguably Phoenix is a slot that could be filled too. But this is much harder than just modding in a cool plane. Bombers can easily become oppressively unkillable or die without doing much. I don't recall anyone making an attempt on a strafer. I suppose your point is that this would be much easier with slower air units and AA ranges normalised down to ground unit ranges, but I'm basically saying that I'm not going to overhaul the unique characteristics of air just because it would be easier.
I don't think any sort of fork is required to explore bombers.