On the one hand, there are far too many zombies already.
On the other hand, something like SWAT4, but not until halfway through.
In most zombie movies, people keep denying there are zombies for much of it, and treat the shambling hordes of undead like shambling hordes of live people that just happen to be decomposing.
But in zombie games, it's generally about zombies from the get go.
It'd be somewhat refreshing to have a game be a mainstream police/military simulator game before the zombie apocalypse breaks out.