Hearthpwn

To make Mecha'thun works you need to have an entire deck of cheap card that can protect you from aggro. You need fast draw as well. And cards that can activate C'thun on same turn he played.

So only way here i can see is Rogue with her cheap spells, Myra's Unstable Element and Necrium Blade. But this deck can be easely countered by destroying blade when Mecha'thun is only card left in hand.

Also it can be played as 'plan B' in combo decks such as Malygos Druid against control decks. Can't see it like a basis for a deck because as druid only card you can kill Mecha'thun at same turn is Naturalize but this is only card to deal with huge threats like Mountain Giant. So if you play both, you can't realize combo for sure.

Mecha'thun = 10 mana, Bloodbloom = 2 mana. 10 mana + 2 mana = 12 mana.