Recorded attempts/Detailed view
The idea would be having the app memorize the last and/or best attempt for each song. This would allow the user to compare the current run with either their best and/or their last attempt and see at which points they failed and what needs improvement. Two points come to mind on how to use this feature:
1. The recorded attempt could show as kind of a "Ghost" attempt while you play alongside your current attempt, giving you real time feedback on where you're improving or what needs more practice.
2. At the end of each attempt, allow a detailed review of the current attempt which could let you analyze step by step where you need more practice (i.e. changing between two specific chords), this detailed view could make use of the recorded sessions (best/last) to compare the two.
I have not found this suggested already but I believe that since Yousician is designed as a learning tool, the star system does not really provide a potent enough feedback. In my experience I can play the same song about 10-15 times and always get three stars but not gold, and there are minor differences in the errors i make in each attempt which make it difficult for me to identify a pattern of what i am doing wrong. The detailed view would allow users to inspect and analyze their performance.