I would really appreciate adding some starting melee weapon in case you run out of ammo and melee weapons in general would be fun) Also, I think we need headshots, like when you shoot exactly in the middle of the zombie sprite, it gets a headshot. Then I guess you need to fix player's hitbox, it's a bit strange that when you rotate near objects your hands push you away from walls and furniture.

Zombies' hitboxes maybe should be smaller to run between them and it was harder to be stuck in them, which is not fun. In terms of shooting, I'm sure, that you'll add better effects, I would love to see some particles like blood, bullet impact particles when hitting walls and some animation of zombies getting damaged. That would add a lot)

Because of small rooms, big zombie hitbox and their amount it's easy to be killed in a corner. Maybe you can make zombies less dense or give them less HP, so that they can be handled with pistols. Right now I can't get to a machine gun with cost of 1500, because I run out of ammo, get stuck and can't run away. Ammo is too expensive I guess and pistols don't have decent damage. And zombies kill really fast, you don't have a chance to kill one zombie to run away from the crowd, if you are stuck in there.

P.S. Maybe you have headshots, but I didn't notice that)