In this post I've included several ideas that I've seen in the forums and some of my own.
-Tank Tracks
-More parts for cars and boats (motors, turbochargers etc.)
-Propellers for helicopters
-Guided Bombs
-Punch holes in fuselage blocks for guns
-Internal Weapon Bays
-Little bit more aggressive AI pilot
-Fuselage blocks should be able to be edited by 0.5
-Fuselage blocks can be tilted to the left or right(Left or Right as in you're are facing it from the front)
-You can edit the width of a fuselage block on each side
-Already made fuselage block shapes that you can customize(hexagons,triangles,diamonds.trapezoid etc.)
-Wider variety of engines including ones you can customize
-being able to change which direction your rotator rotates
-cannons
-activation groups for control surfaces
-multiplayer
@Visify thanks
We already got helicopter propellers rite? just edit the diameter of the blades using XML. If thats not what you're looking for, I've also made some custom helicopter engines that have blades on them that you can use.
Sorry I couldn't put links for some of the ideas. If I missed anything please tell me in the comments