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.
On this, using the correct description of terminology. A scale is not just 'a bunch of notes that sound good' it's actually a set of notes that follow a specific pattern starting on the note that the scale is named after. A major scale has one pattern, a minor scale has a different pattern. And then you have the other types that I can't remember the name of but aren't as important because you don't really use them.
Chords are notes 1, 3 and 5 (sometimes 7, I think) of a scale, played simultaneously. It's not just 'notes that sound good played together'.
Broken chords are chords played unsimultaneously (which technically means all guitar 'chords' in piano terms are broken because the strings all start ringing at different times).
Arpeggios are a specific type of broken chord were the notes of a chord are played in order either ascending or descending. They also follow a pattern, since they are derived from a pattern.
Yousician, please up your music theory (guitar) game.
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.