1.) Historically realistic setups, only one nation per "country" (i.e. early, mid, late war - Knights = Axis, Bishop = Allies,...)
2.) Gameplay that forces goal-driven gameplay (i.e. channel player action towards certain goals that avoid the rampant furball for the furballs sake style)
3.) Update some of the horrible graphics (tracers, hit sprites, explosions...)
4.) Beef up action by adding smart AI aircraft that will do bombing runs, fighter cover etc.
In other words - look at War Thunders simulator mode for aircraft, called "Enduring confrontation". You can see all of my above suggestions beautifully implemented.
I know this will never happen here with all the "missunz" haters and "I wanna fly 2 seconds and then furball with planes that can´t run away from my Spit9"...but hey, you asked.
Litjan