İ didnt get the idea. Why are we killing zombies? i mean are we defending that house? You can add small story maybe so people can relate the idea of killing zombies. Somethink like a daughter inside the house so we have to defend that house.
Player have too much space to move so put some limit like fence maybe. Feeling of tightness may help to increase the feeling of excitement.
Also add more type of zombies and some defense item like turret or pit you can craft and put between waves to help you defense.
My english is not good so i hope its understandable