More advanced music theory
Love the idea, but doesn't have much to offer an intermediate player looking to become more advanced (other than learning complex songs). Adding more scales (modes), arpeggios, and sight reading exercises.
Here's what seems like a relatively simple improvement, in that it wouldn't require much if any new content. Add a few more levels to the Knowledge track, and use the existing "Major Scales" and "Minor Scales" workouts (in Learn -> Workouts). Right now, knowledge ends at level 6.
For example, imagine a new Knowledge level 7 that required playing all major scales at a modest tempo, one octave, each hand separately. Level 8 adds 2 octaves, 2 hands. Level 9 adds minor scales. Some scales do get touched on in the Classical and Pop knowledge paths, but not as a cohesive curriculum.