It is a concept that could definitely be added onto. Maybe variants of the red units AI wise. One might strafe towards you at random intervals forcing you to be more precise with your aim, and another that mimics your movement direction requiring you to pay more attention to them.
You can make good games even with a simple style. It might not be what you imagine, but it's possible to make it fun and interesting in its own way without overthinking it.
There was a bug at some point where the mobs refused to spawn, and that happened for me after round 2 ended and 5 ended. Though it happens sometimes with no specific pattern. I had a feeling that an enemy may have spawned out of bounds, so I was unable to advance further.
The game also freezes if I click off it for too long. Like as I was typing this review it broke on me, and would only fix unless I refreshed the page.