Modern manufacturing technologies enable strange shapes and could produce unconventional pumps. What is good for small scale rockets if turbines and centrifugal pumps have too much tip losses?
Rotary engines have vastly better power to weight ratios and vibrate less than piston engines. They’re also expensive and hard to build (cue modern manufacturing again). Here is one demonstrated in a paramotor, Parajet Cyclone using a Rotron engine. Love the music.