One idea is change the enemy from Zombies to something else.  Zombies are over used, but squirrels are not.  Give the player a goal that the enemy is trying to stop the player from doing.

Imagine the player as a forest ranger with a bag full of acorns.  Their jobs is to plant trees in the forest and the enemies (i.e. Squirrels) are attacking him to try and stop the player.

If you make it level based, the player could have more fun achieving the next level.  On level 1, plant 30 trees. Level 2 requires 50 trees.  While the player attempts to plant the acorns, he has to shoot the unlimited onslaught of squirrels that want to take away the acorns and kill the player.

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

Am I correct in assuming that this is a mobile game? The main thing I would add would be some form of progression like experience or money for weapons. A clear objective would also help. Finally, as with most zombie titles, resources could also play a role; leaving the house to get wood for weapons or defenses, for example. I think the comments beforehand covered any other issues. Keep at it!