Hmm well, there are a lot of little things that poke through but seem more like polishing details like not being able to restart after the game ends, the weapon being at the wrong angle in the players hand, the fence not having a collider, some of the zombies having a weird 2d effect, no music.
Some ideas, it would be cool if there was some way to go faster than just the normal speed. I also can't tell if there is a finish line or if it is infinite? You could add projectile weapons and making the zombies flinch after you hit them to show the player their hit landed and give them a chance to dodge an attack.
Sometimes it's helpful to think of game mechanics you like in other games or look at similar games to find inspiration or see if you can implement what they did.