With little card draw available, the Acolyte and Shield are super important.
Shadow Visions can start powering up the Spellstone or find you a needed answer to an opponents threat
Hemet is useful for getting rid of unnecessary cards and meaning that you can get to the combo without having to wait until you're both in fatigue.
Along with so many others, I have attempted to create a one-turn kill cobo deck utilizing Mecha'thun‘s win condition.
— Since Mecha'thun is the only deathrattle in the deck, he will be the only card summoned from Twilight's Call. The big problem here is that it is a dead card until the end of the game, and if you have more than one Mecha'thun die, two will be summoned, making it more difficult to pull off the OTK.
Most people are using Hemet, Jungle Hunter to thin the deck, and I think he’s important here as well. Doomsayer and potentially Twilight's Call are the only two cards you need in play or in hand before you play Hemet, Jungle Hunter.
Zerek's Cloning Gallery is interesting in this build. It may do nothing (if you have Mecha'thun in hand), but it may also be necessary for the combo. Potentially the biggest problem with a deck like this is you opponents ability to discern what your deck is, and to be able to stop you from pulling off the combo.
–Once you play Mecha'thun, the jig is up and your opponent has every opportunity to transform it. Killing it actually helps us, since we plan to resurrect is later.
The rest of the deck is typical control stuff (not unlike what you would see in a Prophet Velen combo deck).
Let me know what you guys think of the deck, how viable it would be, and any card substitutions you would make!