thank you for the feedback :] i definitely think there's potential to keep the game exciting over a longer period of time, and your suggestions help give me some starting points for experimentation!
Another idea: Balls inside balls
You could use a smaller version of the first perimeter and all the balls inside as the new starting ball after prestige. (This was probably done somewhere before already.)
It could be fun to watch, but regarding graphical processing power and points generation this would get out of hand very quickly …
So you would have to limit it to maybe 1 or 2 "inception" levels and scale up the costs of upgrades and balls after every prestige or reduce the point generated by the smaller balls.
This would be even more interesting with more variety of "builds", i.e. more types of balls and upgrades that do not necessarily have to be maxed or bought at all.
For the player this could require a bit more planning ahead, which might lead to a more involved gaming experience—or frustration.