I really like this game! i think it have a really good potential and i want to play a more polished version of it. I have few ideas that could make the game better:
Maybe the ball could keep rolling when it kill an enemi, to have like a "bowling combot" effect. it could lead to some strategie where you try to line up enemies to kill a lot with only one throw
You could add a way to find the ball more easely, i sometime had trouble to find it when it go out of the screen
Maybe the ball can kill you! it could add a new face to the game where you have to be more careful
You could also add more balls, add power up, healing item etc. There is a lot of things that could make your game a really cool arcade game