At this point I think we can all agree that it is an empirical FACT that a Zombie Mode makes any game better. If you disagree, I submit COD Black Ops, Red Dead Redemption and Arma II as evidence. Furthermore, you are a poopy head.
The obvious problem now is how do you add a zombie mode to an already great game about zombies?
The only thing I can think of is some kind of mechanized zombie, or perhaps adventuring against oversize zombies might be fun. Maybe both of these things might be even better together!
Although, now that I've thought about it a tiny bit, maybe adding a zombie mode to a zombie game cancels it all out. Maybe the extra zombie mode for "The Walking Dead" would just be a series of endlessly repeating mundane tasks that you perform, like work and paying taxes and watching television, all while aging in real time and eventually dying of old age...
...I made myself sad