Really fun game, and the mechanic was awesome to discover and use.
However, there's a few interactions that could be interesting which weren't really explored by the game. The ability to slow enemies and your own projectiles felt like something that would have fit perfectly with everything that's already here. The reasoning for enemies is pretty self explanatory, but with your own projectiles I think it could be a cool way to either increase the difficulty of the game or provide more opportunities for unique gameplay (for example, a boss fight that alternates between two phases, one less intense but it's invincible and one with projectiles flying everywhere but it can be damaged - it would be nearly impossible to hit it normally, but if you could freeze your own bullets then you could leave traps for it during its first phase).
Great work with the sprites, by the way.