One interesting idea is to increase the chanse to draw a 5 star hero by 0,2% - 0,5% per month until someone reach (permanently) 10%-30% chanses to draw a 5 star hero over the years. It would also be amazing if you create a coupon in order to exchange all this months (for all the players) in %chanses to receive a 5 star hero and would be fair enough.
One other idea is to create a Legendary Hero Token only for five star heroes. This token can make an appearance only 1-3 times per year. Or you can earn this token somehow.
These ideas will not damage the competition of the game.
One othe idea is to increase the chanses (in compound) for someone to draw a five star hero each time you draw a three or a four star her.