OK, so I notice that fighters aren't used too often. I see them used as defense sometimes, but sometimes people just build ground based/mobile AA. I've never seen fighters actually attacking other fighters and having a huge dogfight, which I think would be awesome.
I feel that in most group battles, air superiority isn't much of a tactic. Both teams will use attack bombers or fighter/bombers to attack the opposing side, but fighters rarely accompany an army to battle.
I think it would be cool to let fighters have a high/normal attitude toggle, so they could bypass mobile AA defense. That way there could also be high altitude battles for air superiority that took place out of the range of mobile AA. That way, they could also swoop down on bombers, making them more deadly.
Another suggestion would be to lower their build time, so that it's viable to have a decent defense/offense force of them out throughout the game.
Another suggestion, and it might be too radical, would be to change the game so that all AA is basically anti-bomber. Make fighters much faster than bombers, so that normal AA is ineffective against the fighters at normal altitude, thereby giving fighters an ability to more or less bypass air defense. That way, fighters could swoop in at any time to attack your bombers, and the only defense would be fighters of your own. Also, this way, you also wouldn't have to worry about fighters being taken out by ground AA if you're having your fighters protect your ground troops from toads/vashps (I hate the "my-ground-troops-are-getting-pounded-by-aircraft-but-I'm-not-going-to-use-my-fighters-because-they'll-just-die-by-their-ground-AA-anyway" mentality that goes with the game.
Finally, the ability to set up "intercept zones" for fighters (like bombers have "attack area" commands) where the fighters will activate and attack opposing aircraft that enter the zone would be very useful.
Basically, my ideas are to try to change the air game so that most of it is using fighters to gain air superiority, and only then do the bombers move in. I'm not necessarily putting this in as a suggestion, just pretty much thinking out loud (because there's plenty wrong with these ideas).