Wzup mate, im gameplay based dev, so feed about it.
Sandbox its genre with full of opportunities, but here we have just some buttons with same callback. The interes fall very fast, to prevent that you need a bit more interactive, here bunch of ideas:
Direct control the ball - make the challenge for player to reach a section that he want.
Different options of roulette - torque speed, slowdown speed, size of ball. Its sandbox, just give tools to player and they make mad things like ball that cant fit the roulette.
Global goal - not essential but at least something that will be on background, but always visible, its easy way to guide player: "I dont know that i should do, but i have goal, so ill make things to reach it"
Im a bit rude but its my position. Anyway gl with future projects.