I've seen many bullet hell but this one has a nice twist to it! It's a bit rough on the design but very enjoyable to play and each gun feels unique. Well done.
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I hope this doesn't come off too harsh but since you ask ^^... Besides having some sounds (which is a whole other skill that I don't have ^^), the graphics can be improved, maybe some flashier colors, maybe some sprites for the player and the enemies, maybe some knock back effect when a bullet hits. There is actually a really nice talk on youtube about that : "Juice it or lose it - a talk by Martin Jonasson & Petri Purho" that could give you some ideas on how to make it more punchy.
Don't stress about it tho, the concept you developed is quite unique and nice to play ;-)