Prototyping, design circuits and small mechanics components (various material, from plastic to Aluminium, maybe iron but not the better results).
Prototype for testing I think is attractive for a potential client: he has the design, proof of concept or similar. Then needs a prototype well done for final testing purposes and need to outsource the effort needed to make all the prototyping phases.
Firmware testing on prototypes.
I suggest all these skills, that involve physical parts (i.e. components, servo custom parts and circuitry) just because it is almost uncommon and I think that with a minimum investment this is instead "normal" job for a maker.