"The most mutation I can realistically picture would be the zombie not having the usual "Dumber than a cockroach" intelligence. A sense of self-preservation, learning to hunt its prey instead of loudly running at it at the first sighting, rudimentary tool usage (even something as simple as picking up a knife will make it that much more dangerous), maybe communication or being able to lead other zombies.

Best example I would think of is Big Daddy from Land of the Dead. It's established that zombies are starting to learn, but Big Daddy turns himself into the de facto leader. He orders the zombies around and teaches them how to break down barricades with tools, learns to use a gun, discovers that zombies don't need to breathe (and can simply walk along the surface of the river right into the city), and is able to shake off the distracting effects of fireworks. I don't remember all the details, but in the end he's able to actually cause an explosion intentionally."

"Ah so your post is on a subset of more intellegent zombies with more functioning brain matter. Those are plauseable, lol all that zombies can be is plausable as they don't exist. Something had reactivated the brain it is totally possible that the unknown item which reactivates the brain would have more of an effect on some and less on others.

The age of the base person would also play a huge roll in the intellegence of a zombie. The brain becomes more complex as you age and as neurons die off and interconnections are formed so an middle aged zombie would be more intellegent than say a 16 year old Zombie as more interconections between the neurons exist in the older brain."