1, Multiple landing gear options.
Similar to how the custom engines over on SR2 are, There could be a small menu when configuring landing gear that will allow you to choose from a few pre-modeled designs. Instead of wasting parts building your own or having every aircraft stuck with the same stick things.

2, Custom missiles.
This is already done with some modding. But perhaps we could have a special button in the designer menu for the pylons titled "Create Missile" which, when pressed. Will add a thin fuselage block with a standard missile engine on the back. And will allow mirrored attachment points for wings and other details. You could also select the type of missile here. (A2A, A2G, Unguided, or Bomb.) which could change the overall shape of the initial fuselage block/ remove the engine.
(And yes, The game would have to grey out the aircraft you were building on similar to wing editing as to not distract you.)

3, Option to render parts as See-Through.
Self explanatory and useful for canopy things. But who does canopy things. ew.

4, Engine flame designer.
This would be simple probably, But a few sliders to adjust the colour and size of the flame your engines produce at high throttle would be neat. As well as the option to preview these.

thank you for listening to my bob talk.

Trigger Was The Closest