In the immediate future, I think I'll stick with adding new content (more paid song packs and more free songs).
Farther in the future, it might be fun to add a "game" section where users can play short DDR/GuitarHero style games to practice rhythm and possibly learn how to read music.
Also, what's next for Tiny Piano ? How do you plan to keep it simple and yet add features to keep it relevant for new and old users - both ?