A much discussed topic within the DFK community is the excess amount of heroes and the need for a regular burning mechanism.
I suggest having a voluntary hero sacrifice system in which players can sacrifice their heroes for in-game items, similar to PJ (I.e. receive stones, crystals and runes). As this is voluntary with no other upside, I would suggest the rewards being higher than the PJ death rewards.
This would act as a burning mechanism but also incentivise hero quality over quantity.