As it stands now it is a simple and fun game. But what you have here is a solid foundation, No matter what update you have in mind. Pat yourself on the back, Well done! Hope to see more!
If I'm allowed to share one idea. it would be interesting to have an enemy where you don't shoot them unless you're ready. Akin to the Flesh-pound in Killing floor.
A woman who moves slower than the rest. but once hit will sprint towards the player and is a bit tankyer (Spelling?) like the hazmat woman.