I have been playing Supremacy for a few years now and i have always felt that there is no counter to extreme bomber and fighter stacks except for more fighters ( if you can produce them to effect) for example: you could have a level 5 fortress with 1500+ troops and more, but if they person who your going against has a stack of 30-50 bombers, they will annihilate your infrastructure in just one damage phase, granted they have nearly 2 hours wait time depending where they are bombing from. And if they (the adversary you are facing has even 30 fighters patrolling around where they are bombing you from you have next to no way to stop or minimise the amount of loss you can take, that is of course unless you have the fighters to engage his bombers AND fighters. All in all if you are facing an enemy who has an extreme amount of aircraft stacks. Which made me think of an Anti-Aircraft Unit that would make a very valuable addition to the war production lineup of an army/army group. For example for historical purposes/ accuracy they could implement the "Becker 20 mm AA gun" as more of an Anti fighter unit which could deal 2.5 or 3.0 damage to Air units and 0.5 to buildings and other land based units, it could cost about the same as an artillery piece to manufacture. The second could be a heavier "German 88 mm Flak" which could deal 5.0 damage to all air based units and 1.0 to all buildings and land based units, It could cost about the same or in between a light and heavy tank to manufacture. The beckers 20 mm would require a level 2 Factory to manufacture, and the german 88 flak would require a factory level 3 and a railway to be present, much like the railgun.
Let me know what you all think, i for one think it would enhance combat engagements and make certain scenarios that are nearly impossible to counter, more likely to blunt an absolutely massive bombardment, but it could also smooth the edge of overall combat gameplay.