I think that the in game prop engines are a bit unrealistic.
Here's a few ideas.
Engines have to / can be turned on and off. Not just with activation groups, but differently.
instead of stopping, engines would idle.
MOAR SPINNERS. Different option for the prop spinner. None, Cone, WWII, Heli.
Turboshaft engine, for helis. Can be made extremely large, produces lots of torque, takes a while to wind up.
Oil temp, psi,
Torque, I don't know if this is in game, but it would be interesting. Bigger props have more torque.
Customizable prop shape? Experimental idea, possibly like wings.
Basic prop engine. Nothing as far as design goes, just the prop, and basic spinner/shaft. Very light.
Engine. Could be like car engine. Placed anywhere on plane, connects to basic prop engine.
Other engine types.
Jet engines:
Scramjet?
Sartup, as mentioned before.
Oil temp, psi
Others:
Rcs that doesn't require VTOL engine.
Fire extinguisher.
Different engines produce different heat signatures; heat seeking missiles.
Please let me know if you have any suggestions, or if you just think that I'm a freaking moron.

GROUND EFFECT; I NEED IT!