Currently Yousician Guitar difficulty levels go up to 15, and we're working on missions all the way up to that level (15 is extremely difficult, as hard as it gets).
If you're after harder songs there are lots in the Songs tab that should give you a serious workout. Please check here how to search for songs. There's usually a weekly challenge song above level 10 so that should be a good challenge as well.
Remember that if you have finished a path, you can still go back and work on one of the other two paths!