Originally Posted by Mephibosheth
I had thought about this and haven't completely made up my mind. On the one hand, it's always good to inject some variety into the campaign so that the players don't get bored. I definitely don't want to hear, "oh no another zombie *sigh* I'm so scared" in a deadpan that belies the words, so I'll definitely mix things up a little bit. On the other hand, "elite" or "evolved" zombies have always struck me as somewhat arbitrary. If some zombies can be this way, why aren't all the zombies more powerful? What causes the mutation? So, I guess my jury's still out on this one, though it's definitely something I'm considering.
I guess it really depends on how you want to run the game. If it turns out that zombies come in dangerous varieties or are steadily growing in power, than it would eventually prod characters to quest outside of their immediate surroundings to investigate the cause or put an end to it. This is where the arbitrariness comes in, but it could just as easily be a plot device and an inconsistency in the world. Plot devices include things like: zombies have a slight chance of mutation and powerful mutations spread themselves (evolution), evil necromancer is playing with the undead trying to create a super-weapon (intelligent design), surplus of death and negative energy cause undead to be infused with negative energy (natural disaster), or certain individuals or creatures react differently (human nature).
However, if day-to-day struggles are high enough and power-levels rise slow enough there is no reason the campaign can't be about the simple gritty struggle to survive (which, come to think of it, is probably more thematically appropriate for Zombiemageddons).
Now that I think about it, if you allow non-humanoid zombies than you are in effect creating varieties of zombies, it would just be a question of what, how and why. Like you said, something to think about for the future. In the meantime this is shaping out to be a pretty interesting campaign.