Though I'm now noticing I failed to mention - if you do push the ASF into that kind of 'diversity' territory, then you could push for them to be more expensive and not a spam unit, so much so as to allow for other non-t3 units to exist in the air once first ASFs arise.
Also, if you can't imagine it, changes like what I suggested, or other of the 'weapon' type, could push smart micro into the forefront, allowing for players to actually find it more viable to save on mass and energy to build other units (say bombers, but whatever your heart desires). Like the range upgrade for the Cybran could allow for a cybran player to, with enough micro, win with inferior numbers of ASFs. And sure, you'd still have the problem of 'well someone can still spam ASF', yes, but at least you are opening up avenues for different approaches to air superiority game. Like yeah you could spam ASF, or you could micro them and afford yourself more eco or a bomber strike force, or extra ground/navy support with gunships/torps.
Ofc, I'm just thinking out loud here, I'm not a pro player, so maybe my insights aren't deep enough to grasp what kind of situations these kinds of changes could bring.