I think when the set rotates out, Skyshroud Behemoth will come in. It's already good, but shunned, which is beyond me. Ever been attacked by a double-rancored one?
Also, I think anything that coinverts the format of the game is a sleeper. That's why the searchers will convert the game hugely (allowing you to draw an extra card of your choice, for a price, and play it) as well as pack hunt (card advantage), and crumbling sanctuary, for the reasons Zadok explained. My biggest fear is that Eye of Yawgmoth will pop up, especially because I have loudly voiced my pity for the card.