An utterly impractical sequencer that will play a note once a year if you want.
I dreamed this one. I'm not sure what the dream was about. All I remember is an interface that looked like the Pip Boy from Fallout 4, with a touch screen showing a graph: the x-axis was time, and the y-axis was frequency. You could tap to add notes, and they would repeat according to their position on the x-axis. I remember being able to play a note every two weeks, if you want to. I bumped that up to a year, because why not? I woke up and thought, "I think that's worth building."