a chance of dropping 1-3 of a "special" block. Smoothstone, glass blocks. Glowstone etc.
a kinda slim chance of dropping 1 iron block
even smaller chance of dropping 1 gold block or lapis
and a extremily rare chance of dropping a diamond block
To be serious, I'd guess chains. It's a new item, and sorta fits with the mystery of the endermen. As well, chains could be a way to introduce chainmail into the game. For uses other than armor, maybe chandeliers or wall decorations. Possibly used as a pulley system with redstone, /shrug.