This game leaves a lot of room to expand or change it. You could focus on the graphics to change the look of the game.
- You could add more variation in background elements.
- You could also add more/different enemies.
- You might change all the graphics of the game, and change it from an underwater to a space theme, for example.
More work can be done on the script side as follows:
- You can play around with the values for the movement patterns and the hit-points of enemies to find that perfect difficulty balance.
- You can also add new patterns. Try thinking of a move you find interesting, then figure out a good way to script that movement pattern.
- You can also change the effects for the boss creature.
- Likewise, you can add a "death" effect ...