I like this game so far. My feedback right now is:
The menu screen options are too spread out, the difficulty levels should be closer to the centre of the screen like a normal vertical menu.
The controls are too sensitive, the ball feels like ti changes direction too quickly with the slightest movement of my finger. Maybe just alter the controls to be less sensitive, or give the option to change this?
The levels need more variety, maybe different enemy types on later levels, or enemies could go at different speeds?
Maybe include bonus balls the player could collect to give a temporary shield or speed boost?
Sorry if you've already though of all these, I know how it feel to get advice that's already on your to-do list, I get it all the time myself!