Something went very wrong when I downloaded this. It opened on both of my screens, and it kept changing which screen was updating and which would let me click to fire. At one point, my left screen was frozen, but it was the screen accepting inputs, so I was watching my right screen while trying to aim on the left screen.
I have to assume that wasn't intentional, but it was also somehow the most out of control I've felt playing any of these games so far lol.
Anyway I agree with the other comments, I think this is a really good base and could be easily expanded upon. Enemies with different movement/behaviors I think would be a great start if you keep developing!