Pick up (or browse) a copy of All Flesh Must Be Eaten, which includes a d20 conversion appendix. It's basically the zombie RPG.
As for atypical plot ideas:
1) Dealing with a small survivor culture that's built around heavy drug use
2) Searching out a plant in the bayou that supposedly will drive away zombies when burnt
3) The safest building in town is supposedly haunted; the PCs have to either prove it isn't, or exorcize the building, so that the people can safely hide out there
4) A serial killer is stalking the community, providing "offerings" to the zombies
5) People begin believing a virus is responsible for the zombies, and lynch anyone who appears sick
6) There's an old hermit living in a shack in the woods; the zombies haven't eaten him for some reason
7) The community leader is infected and turning slowly, but trying to hide it