2017 wasn't even a real skin - it was a World's skin. Those generally don't count in the skin production schedule because they're selected by the winners of World's. So Zac in reality only has his launch skin and Pool Party that were selected and created by Riot's skin team and not bound to eSports.
I've seen a good concept that would work amazingly for Zac - Tar Pit, and I've expanded on it, at least in text. He could be black and oozy, with bits of bone floating inside him. His blob chunks that fall off him could have piles of bone in them, and his recall could be a play on his regular recall, except small skeletons or animal skulls fall out of him when he gets in the pose, and he quickly gobbles up again. It's a pretty simple skin, IMO, and could fit with like the prehistoric skins.