The graphic style is cute, I like the look of the arena. Whatever lighting+shaders you've got going make for a good look. I bet some collision/rolling particle/trail effects would look real nice.

The gameplay itself is simple but there's something entertaining about a simple physics sandbox, I don't think it's a bad idea at all. Adding a few extra features would be nice (for example: bursts of speed, charging up speed, playing with weight+manouvreability of the balls via pickups, powerups or even preselected powers) and possibly even different arenas with slopes, dips or outright obstacles (not even necessarily static).

I'd reccommend looking at demolition car games for ideas/inspirations, since your concept of going around an arena trying to crash into your opponent has some overlap with the genre and might offer some ideas or inspirations. Some that come to mind are Destruction Allstars, the light cycle games in Tron: Battle Grids and Tin Pin Slammer in The World Ends With You. You might not get anything out of them specifically but it might be a good place to start fleshing out ideas.

I can see this being a fun little party game :D

Hey. I played your game and recorded it on a video. If you want me to delete the video, let me know. Anyway, I enjoyed playing your game.