Informal music notation. Existing systems are fine enough for formal classical notation, but if you want to make a quick rough sketch (e.g., lead sheets in The Real Book) it's pretty cumbersome, and the amount of time & effort it takes doesn't seem commensurate with the sort of document desired.
I've been pondering this off and on for years; I think it might require a pretty fundamental switch in how the data is entered. So far the best I've come up with is, uh, pen and paper...
Have you thought about piano rolls ? Either paper-based ones that you draw yourself, or use software like Reason that has a piano roll for note input or display, nor sheet music. Actually, most non-classic music software has piano rolls.
Sorry for the accidental downvote. I meant to upvote because I have been thinking about (more formal) music notation on and off for years, too, trying to come up with a better format than Lilypond.
I've been pondering this off and on for years; I think it might require a pretty fundamental switch in how the data is entered. So far the best I've come up with is, uh, pen and paper...