Make the radar line of sight as well as distance limited.
This would make the game more 3 dimensional than it is now. Flyers would experience the buzz of flying as low as possible around the terrain to avoid detection when on bombing mission, and when on airfield defence would use altitude more to improve early warning. End result more separation of roles chosen in the game -> less complaints about use of stealth by attackers, and dropping by defenders.
Limited number of aircraft available to each side.
In real combat if you got shot down you were probably dead, so in reality flying would have been less reckless than the way the "head on" suicide brigade play. Maybe air force size of say 5 or 10 times the maximum number of players. Display on screen how many aircraft your side have got left (but not the oppositions numbers). You win if you either take out all hangers or they run out of planes. End result more focus on the aim of the game, and less on just improving your own kill/death ratio.
Minus points if you get killed.
This could be on personal scores, or on final points taken from game at the end, which would encourage more team support of each other in combat.
Variation in the number of hits needed to secure a kill.
Say only one hit to down a top tier play, five hits to down a 5th tier plane. In general that's the way it is. Earlier aircraft were more rugged, later high tech machines more fragile. This would help quell the complaints about the D8 etc etc, and make it easier for new players in slower planes to contribute to the game.
Points for kill depends on who is shot down.
Downing an Ace is worth more to your team than downing the cannon fodder. Most points for downing a Moraf, least for downing an Aspirant.
This would lead to less shooting of sitting ducks in slow planes, meaning less players would give up when they hit the big boys servers, leading to more players in the middle ranks.
Human frailty.
Add screen blackouts if we try to pull too many G's in a turn.
Oil on the windscreen.
Each hit on your plane should put more oil on your goggles, making it harder to see, and making the decision to turn back part of your tactical choices. (And add the smell of the oil also.)
Frostbite.
Make screens and keyboards become really cold at high altitudes.
More Advertising Sponsorship.
Convince the advertisers to pay US real money to play the game, so that we see their branding.
(I've never quit figured out why people will pay money to buy T shirts with brand names on them. THEY should be paid for being walking billboards.)
25hr days.
An extra hour in every day to allow more time to play DF
World Peace
Make it compulsory that all countries settle their differences in DF, instead of shooting down real airliners, or driving real tanks through peoples homes.
A cure for the common cold.
We realize this might take a bit longer, it can wait until you've done all the other things.
A long list I know, and probably others will add more "realistic" requests, but we have faith in you, you can do it. After all, you're Zuperman!!!