Currently Yousician Piano has eight complete levels. Levels 9-15 are still under development, but you can find new content under Challenges tab every week.
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 level 6 and above 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!