imo asf and other aa air units could be made able to shoot at ground units if all aa damage in the game from any plane or ground/naval unit/building was reduced by a huge amount, ex. 100 times, and all air unit hp reduced by same amount (100x), to keep the air vs air and aa vs air game the same, how it already was in original supcom in first place anyways, while allowing air units to shoot at ground without deleting everything in seconds like they do in air, but at least deal some sensible damage
alternatively all air units could be made unable to attack any landed air unit but that's just even worse imo