I want to start the discussion on the addition of airplane designs, that is, the choice in wing type, length, engine model, machine gun number, bomb bay size, etc. Right now, plane designs feel too cookie cutter, clicking a single button to increase the speed of a plane seems.... off. There should be more trade off's, a more involved design, and an actual weighing of decisions which designers faced IRL. For example, British fighters did not need to be as long range at the beginning of the war, but as the war progressed, and the British went on the offensive in the air, long range escort fighters became more of a necessity. This is somewhat modeled in game with research techs, but if the game happens to be paced differently than real life, there should be a way to build fighters which are more akin to early war ones in terms of range (to match the need for more defensive air control) if, for example Germany decides to Battle Of Britain in 1943.

I believe this is one of the biggest areas BI is currently lacking, as the air war was one of the most important parts of the war, with vast resources devoted to it, and for the importance of aircraft, it's research does not match this.