This was cool! Would love to see the full version of this, and since you are taking ideias, here are mine:
-A ranged attack, maybe a power up like it was mentioned in the description;
-Maybe the game could be a boss rush, kinda like "Cuphead" or "Titan Souls", this way having a bunch of unique Boss fights and some more classic gameplay inbetween to not become repetitive.
-Maybe the power up/upgrade bits would be inbetween the bosses or/and as a reward for beating the boss in a special/skilled way;
-The bunny could attack in the direction of the mouse, it feels a little bit strange to aim with the keyboard;
-Abilities that sell more the idea that the main caracter is a rabbit, maybe he can jump, carrots heal him, etc...
- And maybe the ears on the sprite could be bigger and bounce around when he moves, just so his appearance becomes more characteristic with a bunny, since he looks more like a human samurai as of now.
And thats it! Would be really cool to see what this turns into.
(Also i beat the game on easy and normal, Hell dificulty was a bit too hard, maybe instead of only shooting and turning faster, the boss could also gain new attacks on higher difficulties)