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 to the starfish and the diver.