I'm honestly not clear on the aim of the game: is there a scoring zone? Is there a way to miss a return and get blown up? How does one gain score, or win, or lose?
(Forgive me if I missed some demonstration of this in the video--I think that I was somewhat tired when I watched it. ^^; )
However, two ideas do come to mind anyway:
1) Different "characters", each with their own traits (speed, return-volley strength, etc.), and perhaps some sort of "special ability" (split the rocket into two for one volley, or teleport, etc.).
2) Various game-modes, each with their own scoring mechanisms; the two possibilities mentioned above might be examples: deathmatch, in which missing a return results in being blown up, and "soccer", in which there's a scoring zone into which one is expected to direct the rocket.