Bruv,
IIRC, custom tile textures ARE allowed on Main Arena terrains, (been a while since I've looked into it) so it's just a matter of someone making a new underwater brush and making it available for use.
Arlo,
That's awesome, so it looks like the capability for colored lighting is already there, it's just not being used in the Mains? Is that a map setting, or an Arena setting?
titanic,
Flocks of birds (doesnt have to be collidable) - Would add absolutely nothing for the game. Completely unnecessary.
Personnel manning auto ack guns - This would be a nice touch, but might cause spikes to frame rate loss around airfields and towns, etc.
Sun rays - Unnecessary. With Aces High's weather conditions you'd almost never see this effect outside of the occasional special event.
Swaying trees/better looking trees - Once again, unnecessary. Aircraft will almost never be low and slow enough to see this effect, negligible benefit for the ground game, as well
Better smoke/fire effects - I wouldn't mind seeing this. Especially to have variable amounts of smoke so your aircraft doesn't go from perfectly fine to massive black cloud with a single engine ping. Would provide a better visual to attacking craft how bad the target is really hit.
Oil that streaks on your canopy instead of static black mesh - YES. ESPECIALLY since not every engine hit should lead to a giant smear of oil across the windscreen. To avoid costly fluid effects you can even make "oil splatter" decals like the bullet holes and blood splatters. Say, a light hit might throw small splatters across the windscreen that start to build up over time. Worse hits throw more or larger wads on the screen. I think Jane's WWII Fighters used something like this.
Similarily, water droplets flying through clouds - Unnecessary.
Contrails - Would be a nice touch, but questionable on how much of a performance impact it would have. This would mainly be an issue in scenarios and special events since Main Arena fights are usually well below the altitude at which contrails would form.
Concussion effects from explosions - Elaborate. Do you mean more effects from blast radius (IE, near misses working against ships) or purely cosmetic?
Better looking debris instead of plane parts that looks way out of place when they fall - What's wrong with the current effects? I'd assume if I shot someone's wing off it would...look like the wing falling off.
Reflective canopy glass (it looks like glass in external view, but internal view, it's like the glass isn't even there) - How prevalent would such reflections REALLY be? Enough to make rendering them worth the potential frame rate loss?
Depth of field
Realistic/varied puffy ack explosions - So you're wanting to add dynamic particle effects to have ack bursts that don't look the same? Say buh-bye to your frame rate. Completely unnecessary.
Water (like you said)
Dust/water spray when flying close to the ground - Unnecessary, how often are you going to be THAT low?
Better damage effects on objects, planes and vehicles - Do you mean visually or its effect on performance? I'd love to see the more detailed WWI model (IE, hits to the surface skin of the wings disrupting lift) applied to the WWII aircraft, as well as fuel line, control line, oil return, etc. damage. Aside from the issues with oil mentioned above I don't think there's much that needs to be done with visual damage modeling.
New ship death animation - This could probably be made more dynamic without substantial performance loss. IE, if a ship is torpedoed on the starboard side it might capsize in that direction.
Fireballs that rolls on the ground relative to the impact direction instead of just a puff - Probably wouldn't take too much to do, since fire can already be made to follow aircraft through the sky.
Different effects for rockets, bombs, shells and cannons (they all look the same) - At the very least, more variation depending on the size of the boom should be doable. Since it's not a constant effect it wouldn't much impact performance.
Tracers that corresponds with the plane historically - It would be nice to see historical tracer colors varying on aircraft, and probably wouldn't affect performance, but it wouldn't add a whole lot to the game, either, so isn't worthwhile unless it can be done extremely efficiently.