The possibility of creating multiplayer rooms (served based or hosted by players or hosting servers). The characteristics they would have are these:
1) Mod compatibility, with a loading screen that temporary downloads the active mods on the server.
2) Friendly mode: if someone is being annoying with you, you can put it on "Friendly mode" so he can't damage you and you can't damage him but you can still see it as a ghost.
3) Ban and kick options for host or voted
4) Quick events: a menu where you can create a quick event and let people join it or continue on free flight. People can be grouped as teams whenever they want, so you can make up a team and if someone of the team joins the event, everyone do.
5) Airplane editor inside the room, so you don't have to leave the room to change the airplane.
6) Map selection (for future maps)
7) Players limit (up to 32 players)
8) Default spawn coordinates (or random or last from the player)
9) Chat and voice chat (customizable for each player in case of voice chat, as well as "everybody off" option when enter a room). It can also be on the editor, so you can ask things while editing. Also mentions on the game by using @.

Hope you vote this, this may be awesome