Just a suggestion to increase some of the bounty item drops or add them to currently missing:
Frost Giants - Had to fight two of them and neither dropped a head. They are much harder than mountain giants and could benefit from a bounty item.
Frost Orcs - Very rarely drop an item. I think for about 50+ that I killed in the Stormhorns I got like 4 heads.
Ogres - I have yet to see an ogre in the stormhorns drop a head. Had about 1 in 5 drop one in the Hullack. It might be tied to a specific named spawn.
Goblins - About 1 in 10 in the king's forest dropped an ear. May be tied to only specific named spawns.
Kobalds - Seem to have the most consistent and highest chance of drop so far
I could see why maybe limiting the orc head drops and goblin ear drops might be intentional to balance the system and not give a metric ton of XP/gold from something that spawns alot, however, if something could be changed to at least get something each encounter, that would make it more fun.
I feel like harder spawns like giants should all drop something. That got a bit dicey for me, even fully buffed.