This is great. Keep doing what your doing. I have an idea. What if it was cuphead but the characters were rickhead and mortyman and they fight aliens. ;)
Actually I think that you could add gamepad support. This would make it easier to shoot. The gameplay is fun but some more levels would be nice. Obviously you have already made something great but some more content. Even if it costs money would be great. Maybe a level select screen. Still this is a great game and it is great how it is so it is entirely your choice but that is my feedback.