I'm thinking it's not transformation based. That's just lore, and we'll have a finished slime beast by the end.
Though one thing that similar to Anivia and Cho'Gath? I'm think life extension of some sort. Cho becomes incredibly hard to kill due to flat health from his Ulti. Anivia is incredibly hard to kill because of a rebirth passive. Slime in general seems pretty resilient as a body type.
I predict that this ZAC thing is going to be incredibly tanky in some way, or simply hard to just flat out kill.