cool game! i think you can add some specific type of cubes that will able to shoot into the player and he will able to dodge it. And you can try to implement even bosses with shooting mechanics (lasers, guns etc). Also gamemode where enemies moving everytime (not by turns) could be cool. Your own bullets shouldn't kill you by default i think, it will be more intuitive for people. Rebound upgrades after first upgrade should be more expensive, because rebound can easily multiply your damage and now they're very cheap. Thank you for this nice game, it's very relaxing and interesting!
 "Boxtroyer Upgrader"){kind=icon}
 "Contra City"){kind=icon}