The game is a bit crude but have potential. I think making the player smaller and his bullets bigger would make the game feel better as it would make aiming and dodging easier (then you can increase difficulty by adding more ennemies if needed). Also, as previously said, sound and particle effects, even basic ones, have a huge impact on the "juice" of this kind of game and instantly make it feel better.
I made a similar game, if you want you can have a look at it (rating me back would be appreciated too!)