The core mechanic is pretty cool. However, I have some tips for improvements:
- Make bullets massive, at least the size of the characters chest. It makes them easier to see and react to. This may not be realistic, but it's a video game after all.
- Screen shake is a very fast way to add a lot of impact to things (shoot; damage taken) and helps make the game feel more alive.
Hope that was helpful, congrats on making a game!