Vertebral Zac
I heard somewhere that Riot is having trouble thinking of Zac skins. Here is a rough sketch of "Vertebral Zac"
The blob has decided he'd like to be a little more like his makers. By using his good to absorb minerals he was able to compress particles into a skeletal frame. The bones space out as he attacks and in the case of his Q - they even protrude from the goo. Upon death his bones form a skill and cross bones on the ground with his hands and feet scattered, crawling back to his main blob/bone pile.
This skin would require a new model and new animations to be great but could be made with just adding a secondary model within the original and making the original more translucent. To make this even better - add organs.. though I'm not sure what Zac would do with organs - but he does have a mouth and eyes.