Maybe mix zombie theme up with a movable type Paranoia type theme? Something mobile affects any populace it nears, affecting peoples' minds and motives. Instead of becoming complete zombies, those who fall under the thing's influence become obsessed with a goal (kill all lifeforms, destroy all energy sources/food/etc, kill all children). Once devastation has been drawn, the influential object/lifeform/etc moves to another region. Either no one as yet knows what this thing is or what its ultimate purpose is. Or maybe that is known but no one has yet to eradicate it.
This means players will be able to question former 'zombies' etc and also temporarily become one, and you can still begin with the survival motive scenario.
Another option might be to remove the PCs wholly from where they live and abduct them into a new society on an alien planet of some kind where all the former abductees have turned into mind washed 'zombies' - who instinctually kill anyone who behaves 'abnormally'. Aside from escaping with their lives, the plot bonus is that the PCs likely won't be the only beings playing at subterfuge...
If you don't like those, just try mixing the zombie theme with another and experimenting with ideas which integrate the two themes. I'm sure you can come up with several game ideas that maintain your original idea but have plenty of juicy twists.