Multiplayer. You problably get this suggestion alot. I read through some of the multiplayer suggestions, and none of them were specific on what they wanted.
1. I would want a duels dogfight mode. Planes would have to be built by the player. not downloaded or preset planes.

1. AA turrets vs planes. 2 teams of 2 players. One of the teams would design their own AA cannon. The other team would design their own plane. The AA cannons can pick the location they appear at. AA cannons cannot move or leave the ground or they get disqualified.

2. Players can race against eachother on a course.

3. Player escort challenge. One player has to escort another player (similar to the bomber escort challenge) While 2 enemy players try to shoot it down. The attacking players have infinite lives, When they get critically damaged they reappear in a random near location. Escorted player cannot have weapons.

4. Freeplay. A large server where players can fly around as they like. You can choose to be able to take damage from other players or not

5. No XML mods for all of these, and max part count of 200 per player. Players open the console to chat. Usernames appear above players when they are selected in air to air mode.