I really like this as a mentor for people new to ROS, often I find they go too far in one area before understanding a few more fundamental areas. That said, I feel like it would be good to have multiple views of the same tree: a skill based view and a project based view, so that people could choose projects that would teach them that skill (e.g. bash I, bash II, corresponding to modifying bashrc, and piping into grep).
I would also suggest that a few different suggested progressions would be provided for common routes (e.g. electrical focus, mechanical focus, software focus, autonomy focus) (I guess this was also suggested here).