Ideas of Items and stuff:
Leaking Sack(Passive,Buck):"Spits blood"
Griddle Horn(Passive,Griddle Horn):"Careful,it burns"
Bomb-Mouthed Pony(Active,Meltdown):"Flight + Explosive Charge Attack"
Heart Bubble(Passive,Monsoon):"Watery Friends"
Monstro's Snot(Active,Mucus Monstro):"Summon Mucus Monstro"
Burning Priestess(Passive,Fiend Mom):"Mother is enraged"
Mrs.Spooter(Familiar,Retired Familiar):"Spider Friend"
Riding Scythe(Active,Scythe Rider):"Flight + Throwable Scythe"
Urn with a Faucet(Active,Tap):"Reusable blood circle"
Cat Skull(Active,Unpawtunate):"Careful,it's explosive!"
Dad's Key piece(Consumable,Presumed unlock for Dad's Home+ Challenge):"Opens doors"
Jon Jr.(Familiar,Presumed unlock for The Real Jon Challenge):"Bone-headed friend"
Bubble Wand(Active,Presumed unlock for the Dirty Bubble Challenge):"Let's blow some bubbles"

And maybe some new enemies!
Mucus Ministro:Shoots a volley of creep tears)
Techno-Needle:Shoots 4 lasers when in mid-air
Albino Trite:Spawns 3-6 spawm spiders
Ghost Grub:Upon death,each segment spawns a lil' haunt enemy.

If you want to know about Fiend Folio check out the wiki!:
https://fiend-folio.fandom.com/wiki/Fiend_Folio_Wiki

And Preyn,if you were to make a mod based off of FF,what ideas would you have about it?