I would really like a realistic damage model and nice detailed modelling of aircraft (especially cockpit and undercarriage)
Explosions and the way the aircraft fall apart (missile explosions, wings falling off, outright death, huge explosions where the pilot dies instantly, think of IL2)
Good, atmospheric missions with nice clouds, weather and lighting effects.
I would also like self casting shadows like in Lo-mac.
Now, the hardest part. Make the FPS playable even on lower end systems with fairly high graphic settings :)