I mentioned this on the "Future Of" thread, but why not, I'll make an official wish out of it:
A more robust ammunition system. This will come in three parts:
1) Stop averaging ammunition across the belt. Currently, (at least IIRC) the amount of damage caused by ammunition is averaged. So rather than having some rounds do more damage than other, it's all the same. While this is certainly simpler, to an extent it removes the advantage of certain types of rounds. However the real reason for this would be:
2) More ammunition mix varieties in the hangar. For example, the US .50cal is all treated as if it were ball rounds. There's no provision for other ammunition types like incendiary, AP, or API (armor-piercing incendiary). This could be done in one of two ways:
2a) Predefined Belts. Say, the default for US .50cal would be four rounds of ball and one tracer. But you could also have an option (perhaps with a small perk cost) for 4 API + 1 Tracer. Or AP - AP - AP - Incendiary - Tracer.
2b) Allow the player to define the mix directly by adding/removing ammunition on the belt using slots 1-5.
This could also be where Tracers could be turned on or off (as opposed to a check box in the flight preferences as it is now).
3) Tie ammunition into the Start system. Knocking out the Ammo bunkers at a field would restrict aircraft to using the default belt.
This could also feed into a more advanced damage system, adding components like fuel and oil lines, control wires, etc. that can be damaged inside the target.