Well, as an idea, this is good, but like ScotsMiser asked, how slow is slow? The suggestion is, as it is now, a little bit vague.
It needs bit more info in it to actually be good (although, this is good idea, like I said, but it's not anything brilliant, yet.)
And this is bit of a nitpicking, but since 1 block in Minecraft is 1 meter tall and 1 meter wide (Minecraft wiki page), the "Ancient Zombie" wouldn't be that much bigger, than normal zombie.
I'll give this suggestion partial support for now, but if you edit it to be little bit more expansion for it to get full support from me.