Probably been done -- you play AS the Zombie....

Maybe just an alternate mode for a little variety using already created assets/mechanics

Heh in that mode you might sometimes become a Zombie LEADER (Braaaaaainzzzzzz Thisssss Waaaaaaayyyyy......)

Take a look at this
http://www.cataclysmdda.com/ or http://whalesdev.com/ for the original game.

After you get past the initial learning curve and got used to the "graphics", this game offers you a great zombie survival experience.

For me, I don't think I will play (or attempt to make) any more Zombie survival. But that is just me :P. You might be able to get some things out from this game.

Edit: The thing that you want to learn from the games is not the graphics or the perspective but the actions that a person can do when they want to survive in a Z-apocalypse. The main thing is you should be able to do is to make use anything to everything. A crafting system is a must or at least something similar.

You also need to be able to modify the environment or create traps(like previously mentioned) to protect yourself. You need to be able to have an open world or big enough to feel open. In a real Z-apocalypse, you will never really stay in one place. Food will run out and you need to find new food source etc.

If you don't have things like this, then it will not be a survival game, and just ends up being a Zombie shooter, which is fun but have a different target audience from a Z-survival game.

Hope what I said make sense :P