I was suddenly hit by inspiration after viewing one of the new screenshots. Number 7 shows some of the zombies in different lighting, giving them a ghostly and ethereal look. They also looked larger.

To make zombies more interesting. Perhaps you could introduce Megazombies. Randomly during the fight, any zombie can become empowered, glowing and growing like they do in the screenshot. For the duration they have more defense and do much more damage. After a certain amount of time they will revert back to normal zombies. This can happen to multiple zombies per pack, encouraging players to give the individual a wider berth while attacking the rest.

Those are champions! Champion packs receive a slight blue lighting effect to help distinguish them. And they will rock you. I die to champions all the time... It's pathetic.

You'll also notice in the next screenshot the next step, with the champions dead, and what they've left behind.

Those are current health globes.

Did I say something somewhere saying something about them changing beyond this look? Maybe I confused myself.