Combat FS capability...yes...even shooting down ai or other player aircraft. My #1 request because otherwise when I finally replace my dead computer i'll be switching to DCS world...
Associated; Robust support for multi-player even in civil operations. Air-hauler, for example, flying with actual other players would be awesome frosting!
ALL the weather/shadow/cloud/lighting/atmouspheric effects!
Immediate support for add-on development and integration.
Unicorns.