The idea is fairly simple. The zombie survival genre has taken our culture by storm. Creating a world setting for zombie survival type stories would provide a pre made setting for those wanting to scratch that itch. Basing it off an already existing zombie mythos or just creating your own would be great. Ideas would include:

Origin of the outbreak (technological, supernatural, biological, etc.)

Types of zombies that might exist (Reference Left4Dead)

Different timelines and the state of the world (Early outbreak, mid level society collapse, last of the living, etc.)

Other living obstacles that the players might face (Organized marauder groups, paramilitary groups, zombie overlords Etc.)

Specialized character dynamic functions for the human pathos in an unforgiving world.

These are just a few. A lot more could be elaborated but this is something to really consider. Think about it!!! :)