epic game! is takin' me sticked to the screen! anyway here is a little "wishlist" or something like that.. you could implement the game with
-hard mode: zombies do more damage, limited ammo to be collected during missions or to be bought in the store, possible auto-damage caused by a blast radius too close to the character, MOAR zombies.
-survival mode: seriously... ALL zombie games need a survival mode!!!:D maybe with bosses every wave.
and please add different tipe of missions, in the end it could seem repetitive.
continue the great job! 5/5