Write A Melody Over One Chord
I was inspired by the Community Song Analysis Challenge Rock Steady, because the verse is mainly a one-chord vamp. Stevie Wonder's Superstition also has a verse with just one chord!
For this assignment, you're free to write a whole song where one section (either verse, chorus, intro, etc) only contains one chord with a melody over the top.
This melody could be a riff or a melodic line. As long as there is a melody (something like ABAC) over a progression that consists of one chord in some section of the song, you're doing it right!