Earn coins or something to be able to play some songs for free
This would be cool or the badges and points system from guitar bots
another possibility is that for number of stars for each part you play will decrease the amount of play time. Ex: 2 stars could refund 10%. While 3 stars could refund 25%. something like that could work as well.
You get added (amont) time each time you do a song perfect at first try
You also could use the coin to buy subscriptions of yousician