I agree with blacktails...
You could also have:
1: Locks/keys
2: Bouncy blocks that don't kill you, they push you.
3: Black holes
4: Boss levels
5: Give the ball 2 lives on each level and not one
6: A balanced difficulty (slowly gets harder, not one specific impossible level, and dont make the gravity the main reason why its challenging!)
And hell, with all that you can add achievements too!