I remember reading a post someone made joking about giving the ability to change the sound of planes. The sound of the planes does get a little repetative. Why not add some sort of block or gizmo that produces a sound depending on how you fly. Something like the Jericho Trumpets where they only produce the sound when flying at a certain speed. The gizmo/block can be edited to produce the sound at whatever speed you desire and there should be a selection of sounds that can be produced. Just a small idea I had to make planes feel more personal and unique.