OK, I’ve decided to stick to the crankshafts for now! I figured that I at least need to know the basics of designing SEs before diving into the more unique variations.
This is less of a design, but more of a lay-out (albeit incorrectly :P). It has:

• The displacer piston (blue), which moves the working fluid through the engine
• The power piston (white), which transmits the output rotational force
• The linkages (aka crankshafts) for each piston (with their corresponding colors)

The difficult part was laying out every part in such a way that they didn’t overlap or cut into each other (at least the linkages). I was focusing more on the structure rather than the function, so the pistons are positioned incorrectly and will overlap each other when put into Onshape’s assembly!

I’m aiming to create a mechanism that actually functions correctly.