Why not make another one of Zac's abilities also scaled off of HP?
Sion is a tank and his w scales off hp, same with sejuani, and they both bring better cc and damage than zac, infact, alot of tanks have an ability that scales off hp, so why not give the blob that changes size depending on health, actual health scaling on an ability?
Note: I know about the blobs and how much they heal(%hp), I'm talking about an ability that damages(edit: HP scaling based damage.)