Im not sure what the actual code entails so far as there is little feedback, but I have a few suggestions.

1 gizmos to measure forces on the plane. Pitot tubes and so on for pressure, this information is essential really for a game based on designing planes(in my opinion) and simplerockets 2 is awesome for data so i know its possible.

2 transonic-supersonic control surface degradation. I have noticed that when you get to 100k ft and above the control surfaces become very unresponsive due to air pressure simulation(i assume). I would like to suggest using a similar model to deal with transsonic and supersonic control surface problems- as you approach mach 1, degrade the force imparted by elevators in particular(being at the rear of the plane far past laminar flow) introducing the need for a better tailplane such as an all moving tail/stabilitator and so on.

3 rocket engines. ethyl alcohol and so on Bell x-1 bell x-2 x-15 and so on are impossible at the moment because the only engines are jet engines.

4 drag damage, drag is already calculated, add a critical drag rate whereby the structure will disintergrate. For example, high angle of attack at high speed takes a wing off or forcing the elevator up with a all moving tail at very high speed may take the tail off without sufficient structural integrity. Or, diving at high speed producing extreme drag on the wings, takes a wing off.

5 wing thickness. A key element to fast planes was the wing thickness - chord ratio, a very thin wing produces much less drag, but would be significantly inferior strength wise ala #4.

I think thats it off the top of my head.