I own this game on steam and I have talked to you before about maybe adding music to the game but in the full version I think it would be cool if they were steam workshop support for making custom powerups. Also, the game gets boring after playing for a few hours, maybe some sort of challenge system where you have a few challenge slots and when you complete one you get a new one and for completing them you get some cosmetic settings, or it could be just for achievements.
Maybe one that that repels nearby enemies in a blast-like manner or for a period of time, I've noticed that I often die it's because I'm surrounded by enemies so it might be a separate mechanic you use at any time by pressing a certain key/button (I've played on both Keyboard and mouse and controller and I think controller is better (My wrists hurt less)) kind like how the shield is a power-up bit it's a separate mechanic.
Also unrelated to powerups, is there any chance you could add 360 degree firing for controller. If you're not up to that because it would be an unfair advantage, you could always make a separate leaderboard for controller runs. You don't have to accept any of these ideas, just thigns I've thought about while playing.