First of all, the game needs a "game over" state, badly. Winning is optional, but losing is a must; otherwise, it's just not competitive at all, like an interactive screensaver.

Another obvious improvement is adding content—diversity is king. More enemies, more attack patterns, and probably the ability for selected enemies to shoot back at player. A progressive increase in difficulty level goes in the same bucket: later stages of the game should bring bigger waves of enemies, faster spawn times, and so on.

Adding sound effects is probably the simplest enhancement. Refer to Chapter 7, Writing a Flappy Bird Clone, for details. The same MultiAudio class can be trivially reused in this project too...