Let's look at this another way, by analysis of current boss drops.
Each boss drop has had a unique way to act as a progression gate.

antler - crafted directly into a tool

Key - non crafted item to directly unlock an area

wishbone - non crafted item that needs to be equipped to use <iron itself also acts as a gateway due to needing iron to mine the silver>

Tears - part of a recipe to create a crafting station to produce items needed to process/craft weapons/armor/food

Now the tears really already produce the padded armor that is really the transition armor for the Mistlands.

So logically the Yag drop should be something to work with resources from the mistlands in some unique way in order to make the next level stuff.

We have no idea what might be in the next biome to even process. We have no idea if things will be changed in the plains to sort of nerf the plains stuff now and make it for Yag... just like how they swapped things around so you can't make serpent stew without iron.