Mobile sound points. Currently the sound origin is fixed, while the gun can swing in a turret. At certain points this makes the gun sound muffled, because the sound origin is behind you.
Death to sprites! Particle effects can be used to replace every hit sprite with a small performance hit. Using particle effects for muzzle flashes would also be a serious improvement.
Square smoke needs to go away. How about a more rounded smoke with ragged edges instead? It would be especially nice for the flak blasts.
Better bomb/tank gun blasts. UT2k4 has a killer particle effect for both the muzzle blast and impact of a tank round. Hardly causes a burble in FR too.
Believable tracks for vehicles. The one micron thickness of the current tracks looks horrid, and to boot the tracks aren't animated. I'd like to see both animation and thicker tracks. If possible, model the individual track segments. Also, tires need to be more round.
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Flakbait [Delta6]
Delta Six's Flight SchoolPut the P-61B in Aces High