The governor will be the evil AI that will hopefully make this game difficult. I suggest to have it change the weather, e.g. create a hailstorm that destroys your harvest and create minor natural disasters like a fire. The zombie hordes will challenge the player, even if he has built a nice safehouse and has stored a lot of tools and resources and the weather and disasters will simply add to that. I don't care much about the plausibility at this point. Times without rain or excessive rain are plausible on their own, so is an exceptionally cold winter. Lightning to ignite a fire is plausible too and so is a hailstorm. Problematic is the fact that all of this is directed at the player.