My only issue with all the suggestions here is that, with the current star system, it is already possible for players to achieve 15 stars on their heroes.

Once a hero reaches 15 stars, that is lost opportunity cost for Tynon, since you would not have to spend gems anymore on that hero.
I believe not all players or heroes should reach 15 stars. Stars are the most luxurious thing (and most expensive) of all the features in Tynon.

Everyone wants to make the star system easier, and I don't believe this is right. It's fine the way it is.