- Cornered into a small settlement, you can either assist the remaining survivors or make a run for it, leaving them to die as a huge horde swarms the town.
- Your exploring a science facility when you come across a infected scientist strapped to a chair. He saids he found the cure, blah blah blah, and if you help him, he'll tell you about etc. etc.
- A barricaded town has popped up, and your tasked with either helping them prosper or killing them off and using their goods to help you survive.
I do get what you're saying, but you have to look past the set pieces and see what these tasks would entail. The scientist example is the only one of those three that appealed to me, as it would certainly shake up the formula and would introduce some depth to whatever it was that you were doing. The other two example would just be killing zombies or humans. Not much variety there.
The things that have made past zombie games so successful weren't the depth of the game, but the gameplay. Why did people play Left 4 Dead? To have fun killing things. This isn't Beth's strong suit, and even if it was this would be 'just another zombie game.'
As for the variety of zombies, you can only do so much before they aren't zombies anymore. Again, I'll use Left 4 Dead as an example. Look at the zombie types. What more could you do? You could change the way these zombies function, but you'd still have the basic 'generic, strong, support' zombies. If you tried to enhance them too much they wouldn't be zombies anymore, as they would have to have some kind of intelligence.
EDIT: I'm sorry if this post was incoherent, but I'm trying to say what I'm thinking without writing an essay.