There are a few things that would, in my opinion, need to do.
-Add an actually goal
-Not put you through the same few levels non-stop, so basically just more levels
-Fix the bug where balls get behind the walls
-Add a community area where people can make their own levels
-Add an upgrade system of sorts outside of each level, like an overall upgrade shop you get points for from beating levels. Shop items could be like background customization, the 3 upgrades but permanent, and ball customizations.
Loving the game as it is though, just completed everything already, probably 5 times over.