Hearthpwn

QUICK EDIT: I do NOT think this version is better than the standard mecha'thun druid list, it just has a better matchup against other combo decks, due to the ability to high roll and combo out so fast, it's worse against other decks, so it really depends on what you're up against.

So the premise of the deck is simple. Draw our combo pieces, and play Keening Banshee to burn our remaining cards and win the following turn.

I'm running a single copy of Juicy Psychmelon for the sole purpose of tutoring up our Mecha'thun, increasing the consistency with which we can play the banshee. It's probably the only card in the deck that's awkward in terms of finding the time to play.

I always found mecha'thun druid to be a little awkward, mostly because if you put in too much card draw, you run the risk of fatiguing yourself, so it's a risk to run UI, which is one of druid's most powerful cards, since you still have to play all of your cards to combo off. Additionally, it can often be awkward finding the time to play all of your cards. If you're about to die, cycling your entire deck with Gadgetzan Auctioneer doesn't help much, since you'll still need a turn at least to get those cards out of hand. Once you have your combo pieces in hand, Keening Banshee is effectively an auctioneer that draws 3 for ANY card you play, and since you're going to win the following turn, it doesn't matter that you can't play them.

I cut one copy of Gadgetzan Auctioneer for the banshee. Until you've drawn your combo it's obviously worse, since it can't draw you into your combo, but unlike the auctioneer, banshee has the benefit of burning the cards so that we don't have to waste turns getting them out of our hand once we already have the combo. 

Honestly the most awkward part of the deck is making sure you can remove your minions before your final turn. Against other combo decks I highly recommend saving a Naturalize for your own minion, since they often won't have anything for you to trade into.