As the title states, repentance plus (and fiend folio) support would be really nice and be quite a cool addition to those who have it enabled

Here's a couple suggestions for what the hearts do for the Lost/T. Lost:
Broken Heart (the heart unlocked along with T. Isaac)
-On pickup, the broken heart turns into a Book of Belial wisp
Baleful Heart (the heart unlocked along with T. Lost):
-On pickup, the baleful heart turns into a Bible wisp
Deceiver Heart (the T. Judas heart) doesn't need a new wisp because of how it works (it splits into a half red heart and a random pickup (bomb, key or a penny) when approached)

For fiend folio:
Immoral Heart
-Creates a potato peeler wisp for every 2 immoral hearts obtained
Morbid Heart
-Creates a Duke of Flies wisp (from Delirious) for each full morbid heart obtained