It always disappointed me that humans haven't really managed to make the rotating detonation engine work. Its theoretical efficiency and power density is really high.
Now I'm imagining something like a fidget spinner combined with a Nuclear Pulse Engine[1] and it's awesome because I'm imagining that it can provide both propulsion and rotation...but I bet we need some great advances in material science before anything like it could exist.
A Wankel was what I thought of first, since it's the closest we've come to a rotational explosion engine, but I'm a space nerd so the combination of rotation and propulsion could make for some interesting applications related to space travel.
1.https://en.wikipedia.org/wiki/Nuclear_pulse_propulsion