I'm writing another song to procrastinate from starting on class preparations. The harmony is a bit more sophisticated, at least I use more chords, with a melody almost as simplistic as my first one. I had avoid having the exact chord progression as in my first song. Certain chords are addictive, though. I learned about the tritone substitution recently, and it just sounds very cool, but then I realized I didn't have to use it every time.
Melodies can be rather simplistic. Look at one note samba or c jam blues, or thelonious. You can repeat the same phrase and put a different chord under it.
The process is infinite, because even after the song is written the arrangement needs to be refined.