Zero probably could add another class, but I'm sure hes working on other stuff as well. Though I don't see why a zombie couldn't mutate and have the ability to blend in with the world's surroundings. If anything, the zombies from ZM are the equivalent of zombie bosses in L4D on expert.

I have to admit though, having two predator classes is somewhat pointless. Keep the one that has nearly full invisibility until you shoot it's head off, and replace the other one with an entirely new class.