In my opinion what would make zombies more interesting would be huge herds of zombies crossing the map. Yesterday I was chased by 5 or 6 zombies when I approach a city and it was incredible, adrenaline rush somewhat close to PVP, since they were a lot and I only had a couple of bullets. I had to run around and take them 1 by 1. If instead of 6 they were 50 I would think twice before going into a town that had been overrun. The a group of zombies (15-80) could just start spawning somewhere and head in a random direction. No need for better AI for that.