Sounds like the first thing we need to figure out is the Zombies. What kind are they? How smart are they? Are there multiple varieties? Do they have any goals?
I'll ponder this today and come up with some detailed ideas.
This would be largely dependant on the game style, wouldn't it? If squad based, having a line of zombies limp towards your character as you shot them one at a time, counting your bullets and making headshots would be extremely tense. First person games don't have quite the same feel. It's harder to get the players to play right, since they can more easily headshot the crowd, or run through it. You'd almost need trackstar zombies in a FPS environment.
(for FPS, consider sprite based rather than 3d models... you can get a lot more baddies on the screen)
Mutant Zombies? Rather than traditional undead, mutant zombies are the result of a gene-altering pathogen. Several brands of this and several levels of contamination could yield a nice variety of enemies. It also means that the mutagen came from somewhere, and yields a plot. I'm not thinking anything crazy like RE or L4D, but think of a mutagen that causes zombification (loss of pain receptors and intelligence, mindless rage), but also causes bony plates to form on the skin. Initially, they'd just be a high DR zombie, but could quickly become horrific, with skeletal extrusions all over the place. Progression: (Zombie, DR Zombie, Plated Zombie, Spike Monster (high DR and armor piercing), and the rock zombie that can't move, but is nigh-invulnerable)
Infector track:
Zombie
Fast Zombie
Puking Zombie
Plague Bearer (think bloat here, only not quite so ridiculous. It would be fast, and any blows would burst the swolen nodules releasing retrovirus in a cloud of goo.)
Basically, take a cancer and a virus and an overactive stem cell disease, shake it up and set it lose. 99% of zombies would be boring, but this gives the director a chance to throw a unique/interesting creature into a dark building for high suspense. It also makes 'old infection' areas more dangerous. The important thing is to avoid the RE 'guy morphs into a locally appropriate unique monster in a matter of seconds' thing.