Show accuracy on various notes and chords
I see that you track the number of notes / chords that a user has performed. It would be nice to show a breakdown by each chord and the number of times that the chord is hit or missed (perhaps even show timing information). I am not sure if you actually track this on the backend, as I imagine this is a lot of data (IE every note/chord that every user attempted, and whether they missed it, perfected it, late, early, etc).
I think there's a lot more detailed information that you should be collecting and reporting to paid users.
Which chord do I miss most?
Which chord do I perfect the most?
Which chord transition do I miss most (G major -> F major)
For notes you could show fretboard positions that are weak areas? Or arpeggio's, patterns that are weak.
Once weaknesses have been established, songs or scales can be suggested that help improve.