To give the chance for new players to find the OLD heroes of the month, I propose to reintroduce them every time a new hero of the month with the same color is added:
Hero of the month violet:
Add the new hero and give a chance to find Hell too.
Hero of the month blu:
Add the new hero and give a chance to find Athena too.
This will encourage the new players! Maybe will be good give to old players that have the old hero of the month only the chance to find the new one so that proposal does not hurt the old players.