Speed, sonic booms, and smoke in general
A sonic boom that varies in shape and size at different altitudes and heights, should be easy with the games current system and funky.
air thickness and drag should be modified to be more realistic, most height info in simpleplanes isnt incorrect, its just compressed, id experience difficulties at 60k ft that i wouldnt in a similar aircraft irl, i think they should get increased by about 1.5, so id experience current 60k at 90k
if you cant find a way to make contrails, gun smoke, or anything similar, how about a smoke block? and one that can be modified in shape and size and colour, fe: stationary smoke that moves with the aircraft and leaves no trail, smoke that is continuously generated then slowly deleted for contrails, and so on and so forth.
i think these are small and easy additions that can bring alot to the game experience, anyway, thanks alot for your hardwork over at jundroo games, and keep it up <3