Some early ideas:

Dynamics exercises. Capture amplitude changes, so users can practice crescendos when using acoustic guitars.

Solve the reverse problem, but for singing. Given a song, extract it's melody and replay as a karaoke with scores.

Guitar Pro compatibility is more important than you think. You'll probably have to differentiate yourself from Yousician.

Cheers.

Thanks for that. Just in case you missed this point, you can already import Guitar Pro files.