I like the ideas of master zombies becoming some kind of horde gatherer. Gathering most of the green zeds around them, it both makes them able to use their special ability to more effect, as well as discourages trying to run them down in smaller vehicles as the zeds can potentially stop the car dead.
Make the green zeds prefer following the master over the player, they get to be a protective wall, absorbing bullets and making melee risky, while the specials the master transforms become the threat to the player. Allow the master to weaken his shield himself, but have to fight special zombies a few at a time, or go in quickly, killing the weaker green zombies before they become dangerous at the cost of more dangerous, aggressive tactics, and being in the middle of a horde at close range when the master does go down.
But they might need to get a little smarter as well, give them a short range teleport, like the cougar’s leap, that they use when they think they are about to be run over or an explosive is nearby, or make them try to stay under a roof when they hear a vehicle. Few players are willing to damage their vehicle by driving through a building. Force them to fight it on foot or be willing to spend a lot of batteries on repairs.
And then, with the discussions on static spawns and a thought I heard of making greens randomly mutate to specials over time, perhaps having a master among the population can increase the mutation rate of surrounding tiles. Make the player NEED to kill them.
We need to feel a sense of accomplishment killing a master.