Clockwork Elemental Overheal

Game Plan

Turn 1-5 are Overheal Priest: try to get a large and buffed board. If that doesn't work out, turn 6+ is big minions with nice battlecries and deathrattles, including some Elemental and Undead synergies.

Reno should be active on turn 8 because you draw your duplicates on turns 1-5.

Mulligan

Mulligan for second copies of the 1-4 mana cards. In particular, a second Fan Club or Hidden Gem is great when you're on the Coin.

Replacements

I'm relatively confident that turn 1-4 are optimal. For turn 6+, it is hard to say: it felt powerful, but I didn't play enough of it to know what the most powerful cards are. So far, none of them feel essential, although Thunderbringer into Neptulon did feel good every time it happened.

If you are missing some of the legendaries, go ahead and substitute them with any other 6+ mana card you like. Avoid duplicates unless you take out both Reno and Maruut.

I had Heartbreaker Hedanis in an earlier version of the deck, but it was too hard to get him overhealed: the deck doesn't have all that many heal cards, as adding those would weaken turn 1-3, since you might no longer draw the best cards for those turns. Ambient Lightspawn is way easier to trigger, as you typically get 2 or 3 procs on turn 4.

Animate Dead is another card I dropped: it is more likely to resurrect minions summoned by Heartthrob than Heartthrob himself. Also your opening is a lot weaker if you don't always have Fan Club on turn 1.