high alt and maneuvering contrails, rocket and jet exhaust trail, exhaust flames incl. for jet engines, firewalling burst of exhaust smoke
fast enough sound system for correct rate of fire sound (eg 50cals that don't sound like they're firing at 600 rpm)
historical color tracers
pixel shading
reflectivity mapping for skins
more physics
more damage model details, e.g. structural and aerodynamic effect of a tater hole in fuselage, control cables vulnerable so e.g. .303 banks are more competitive
destructible greenery and buildings
random urban areas for better ground combat
multiple roads between bases and strat and random urban areas
more terrain topology detail
submarines
FPS element for map room capture
Korean dogfighting arena