As much as I love the idea of being able to design conventional aircraft, I'm not sure if adds any meaningful gameplay decisions to ground unit design. Bringing in a quote from another thread:
Extending this argument a bit to conventional air power, I don't see that inserting another ground unit type adds much when the current mechanics support roleplaying <vehicle> types as aircraft. For example, strategic bombers could be modeled as artillery, with strategic AA or dedicated interception squadrons as counter-battery. CAS and its vulnerability to ground fire is easily mapped onto light vehicles in frontline attack. The occasional hits AP weapons inflict on heavier armor can be thought of as organic AA that's below the resolution of the unit designer (e. g. MANPADs carried by an infantry team otherwise equipped with PWs).
From this perspective, the current "AA" role is more like a souped-up version of strategic AA or missile defense that may not be able to engage anything below several thousands of meters.
Alternate take: we already have a ground support fighter (GSF) mechanic, which does occupy a space between ground-to-ground and space-to-ground, and the fact that no one is really saying "let's take this out of the game entirely" does indicate that it adds meaningful gameplay - or at least that people think it
would if it worked worth a damn. Mechanically, the idea of a flight-capable unit type would be to keep the same or a parallel set of mechanics, and just reframe that in terms of a ground unit class to eliminate most of the reasons why the current GSF mechanic doesn't work (unreasonable micro, poorly-scaling logistics, clash between AA and ship-to-ship combat mechanics, etc.).
In terms of gameplay role, I think the comparison has to made against the heavy bombardment (HB) component type as this is the parallel the existing GSF mechanics draw (sharing the same targeting rules). In this case, a GSF/air force mechanic provides the ability to strike in a HB-like manner with non-bombardment weapons (AV, CAP, AC, etc.) while only being countered by the presence and firing of enemy AA units (providing an interesting defensive decision - how much AA, and how to distribute it? - which goes beyond the existing aggregate damage mechanics and adds a new dimension to force composition). Presumably this is balanced by any number of factors - build cost of the base unit class, fragility in armor/HP, large transport size representing extensive basing requirements, etc. I think there are plenty of options to make these an interesting and unique unit class from the mechanics perspective which is not too dissimilar from the current GSFs.
Besides, even if it is not mechanically distinctive, I think it is an important roleplay niche to enable air force modeling in a way the current GSF system has so far not lived up to. As someone who does like to model his ground forces in some detail, I don't like the idea that a LAV is both anti-tank and "anti-air", simply put a FGM-148 Javelin and a FIM-92 Stinger are not the same thing at all beyond both being man-portable launched missiles, nor does it really work for roleplay consistency if a LVH+CAP is an APC but a LVH+LAC is an A-10 Warthog equivalent.
The challenge of course is no matter what is done, much programming is needed to implement this on Steve's part, including revising the ground combat rules to accommodate this new unit class which could run afoul of the "reduce exceptions" principle for C#. As Steve has not much commented on the subject I am not sure how he feels about that.
As an aside: I would still like to see one mechanic from the existing GSF implementation preserved. While fighters should play by the same rules as all the other spaceships, I'd like to see the ability to link a squadron of several fighters to a single FFD unit preserved, so there are sensible alternatives for naval bombardment besides using the biggest battleships you can find.