Gregory's ServerAn outlandish engine concept I'd like to try and prototype someday is a hybrid compressed air/solenoid engine.
Solenoids are simple electromagnetic mechanisms that people have used to make little toy piston engines. They also get quite hot when cycled on and off.
Compressed air engines are simple and clean, but suffer in power density because expanding gasses get cold, and thus lose motive power.
So a little engine which combines the two would be fun to tinker with.
I'm under no illusions that this would be a *good* engine, just that it would be an interesting way to balance the shortcomings of both approaches towards something symbiotic.