Apart from the fact that I don't feel like it implements the theme at all, this game is AMAZING. You definitely need to make it into a full game.
Here are some improvements you could do:
- more boss attack patterns and maybe weaknesses to an element so that it deals more damage.
- better hints for recharging ammo and where the player needs to go
- interactable npcs
- upgrading each "gun" by progressing through the story or beating a boss would be really nice
Overall, a really pleasant experience!