What i mean is that yo can chose solid/liquid the weight and how much thrust it produces along with maybe the chemical compostion or some other stuff then you can name the fuel and either use it on your craft or share it with friends! If you made the fuel a higher wait it might increase the thrust for instance so you couldnt just make wait 1lb and thrust 99999999999 newtons, it would calculate how heavy 99999999999 newtons would be.

tl;dr
custom fuel types
weight+ = thrust+ or something like that
plus selecting liquid/solid/some other types of fuel.