What you would do about the old legends to make them be in the same level as the new legends? The obvious answer is buffing them, but i would do something more. Here is my idea:
I would buff the old legends, increasing their stats and upgrading their moves. But since they are breedable and a monster with high stats, great moves that is breedable could break the game, I would make them unbreedable. But, since it would be unfair make them unbreedable, because the old players obtained them by breeding, and the new players will have to obtain them by spending gems, to make things fair, I would do this: Every new player will receive ALL of these legends that are now unbreedable, and every player that doesn´t have a certain old legend will receive this certain old legend(for example, a player doesn´t have Darkzgul, so he will receive Darkzgul).