I think this idea can be expanded into a really cool game with a couple of changes.
- Your bullets don't disappear when they hit the enemy. Whenever you fire a shot, you just add another moving obstacle to your path, even if you hit the target. Also if they bounce immediately after hitting an enemy, the player won't be able to walk right next to the enemy before shooting.
- Add diagonal surfaces all over the place. If the bullets just bounce back and forth infinitely off of straight surfaces, the mechanic isn't being used to its fullest potential.
- Most importantly, give the player the ability to hit a button reflect bullets in the direction they're facing. But only their own bullets, not the enemy's bullets.
And boom! You have a mix between a top down shooter and Breakout.