Enderman's Hoard

I was thinking about what Endermen do with the blocks they pick up. Why are they picking them up? What happens to them??

What if they test each block for certain qualities, those that don't pass are put down again in the overworld. Those that do pass become "ender touched" and are transformed somehow. (Maybe have purple sparkles). Those blocks are taken back to the End Forge to do something with, maybe extract the stuff and form and Endercube, which opens a doorway to their lost hoard... or maybe an Elder Enderman (possibly a new boss?)

After the dragon is slain, maybe an enderman leaves an endertouched block near the player and picking it up gives a vision/clue.