Ideas for new levels will come once you come up with new mechanics, or just more obstacles in general.
I'd say to add a level with a mechanic that matches the games simplicity, like color. I noticed that all obstacles are red while the ball is green; Make a color switcher that allows the player to swap between red and green at will, with the obstacles and the ball doing something different if they are green vs red. Maybe obstacles don't stop you if they are green, maybe you can now jump over them if the ball is red. Adding a third or fourth color only expands the arsenal.
Feel free to check my game out as well, I could use some feedback: https://wasabiiii.itch.io/grass-elemental-demo-demo ; In it I've been experimenting with new battles and "levels", seeing if I can present the ideas I've had for a while intuitively