"If you thought Zombies on your lawn was bad...
Wait till you see the rest of the world!"
Dun Dun DUUUUUUUN!
I'm thinking Pokemon structured game. You play The Gardener (Male/Female). Collect the Plants to defend youself (AND THE WORLD) from the ravenous zombie hoards!. Maybe some kind of capture mechanic to aquire new plants, and then tower defense gameplay to protect your fellow neighbors (AND THE WOOOOOORLD!!!!) and liberate the towns you will visit.