I really like realistic planes, and while they are possible in SimplePlanes, I think the following suggestions could make them a lot easier to build and less laggy.
Smallest (most likely to get implemented) to biggest:
1. Switch Cover Switches
A cover for your important, "do not touch" buttons! Pretty self-explanatory, a safety cover for switches (also make sure you can't reach through them in VR)
2. Linking: A faster way to create functional switches
In the same way you can link a wheel to an engine, you should be able to link a switch to different parts (i.e switch to engine). You could also link parts to switches (i.e. Engine to multiple "starter" switches). This removes the tedious process of editting XML for everything you want to link to something. Technical side: It creates a new activation group (like you would in XML) under the parts you choose. If you link a part to multiple buttons/switches, it creates a variable to make sure all are activated before the part turns on.
3. SCREENS: Putting funky screens behind us
A screen component (like seen in the Wasp), with multiple options consisting of:
Informational (like in the Wasp)
Radar: Shows enemies in correlation to your position
Map: A top down view of the world, with your plane in the center
Cameras: Shows what a certain camera sees
Screens could also have a way to add buttons, in the same way you can add control surfaces to a wing. These buttons could change what the screen shows, like switching cameras or even switching weapons.

In conclusion, while most of these things are possible with funky trees, implementing these features into the game would make building realistic airplanes easier, and make realistic airplanes run faster.