To give some ideas on how to do a Zac skin, his outward appearance doesn't have to change that much

Let's face it..."Special Weapon Zac" is as underwhelming as skins comes. It's Zac...but purple. With some purple tendrils randomly thrown in. Thematically it's not inspired by anything, it doesn't change anything in such a way that warrants it's price, etc. Many who like it would like it to justify their purchase of the skin.

However, such a skin could have worked had his abilities been altered more significantly. Zac is supposed to be a slime monster, he should have the ability to alter the shape of his body to varying degrees. His skins can be based off that concept alone!

For instance, since you have a tentacles/spikes theme going with Special Weapons skin, his Q instead of being a punch, could have had his arms morph into giant impaling spikes or tentacle masses. His W could have been wiggling tentacles flailing around. His R could have ditched the blobby-look at made him bounce around as a writhing mass of tentacles and spikes instead.

Other skins could be something like a more advanced and intelligent version of Zac, like "Perfect Zac" or something where his initial outward appearance doesn't have to look all that radically differenty, however he is capable of shaping his body which much greater expertise. In this case, rather than the default Zac's more "primitive" methods of attack, he morphs his body to attack. His Q replaces the arms with swords and spears complete with a metallic stabby sound. His W merely has him produce saw blades or more swords which swing around his body. His E could have him morph into a giant bullet mid-air. His R could have him turn into a spiked ball or something.

TL;DR - Your "Zac but purple"-skin could have worked. If you are afraid of making a Zac skin too expensive or resource-demanding, his initial model doesn't have to go through too drastic an alteration. But all you have to do is change the looks of his abilities.

Also why do I have a feeling that when Zac FINALLY gets a skin...it's going to be some sort of legendary skin so that Zac players only have "Zac but purple" or a frikkin Legendary to choose from?