Do you have plenty heroes that you deemed not so useful?
Or
Do you have an "almost perfect" dream team which you lack some particular heroes that you never get?
I would propose:
Hero exchange event.
It doesn't have to be 1 on 1.
2 or 3 hero (of the same rarity) to 1 hero of your choice (of the same rarity) will do.
Scrolls that already invested in the hero to exchange will not be refunded.
Devs can also add some additional costs such as wyrms or dices, as long as it still logically worth the value.
Legendary to Legendary.
Epic to Epic.
Rare to Rare.
Please feel free to share me your opinion.