I know the zombie AI is in it's infancy but one thing I would love to see is the zombies moving around. They are so stationary that they feel static. I would like to see them slowly lumbering around.
Think of walking dead season two.
In the opening scene, if I remember correctly, a helicopter flew over a big city and started the zombies following it blindly. In the finale they end up at the farm.
I think this would add a lot of gameplay to the game because you would find zombies in the wild as well as cities and they would be less predictable inside cities. You might run into a horde on accident and have to blast them all just to survive. That would be frightening and awesome.