Have an in-game program that people can use to make their own parts. In this program, people could build their own parts with its own part properties that are saved as a single part that can be loaded in the normal craft workshop. For example:
Let's say I want to make a complicated landing legs that uses piston and hinge like parts to extend and retract, but I don't want it to use a slider. So I go into the part workshop, make my landing gear the way I want it, and define its retraction state with a book value. If I want, I can also make some of it's properties, such as retraction speed, and whether or not it starts extended, customizable in the normal craft workshop.