The VTOL parts (engine, nozzle, and RCS port) have been in the game practically since launch, and are missing a few critical features.

First, customizable/selectable inputs for the RCS ports would be fantastic, as they would allow for far more useful placement and usage of the nozzles if they weren't dependent on what input the game "thinks" they should have.

Second, the ability to "pair" VTOL nozzles to a specific VTOL engine, in the same manner as resizable wheels/car engines, would be an absolutely fantastic feature, as it would allow for far more complicated uses of both types of part.