A cool feature would be a piano key color fading transition when you hit the key, which will adapt someone's memory more quickly to being able to complete the sequence with multiple retries.

Should be as simple as adjusting the color interpolation in an animation on a piano key to its gain (not sure how low the sound API is for this, but you should be able to track the key's moving average on amplitude over 10 frames and bound it to get a fade effect that takes the key press strength into consideration).

Perhaps you can even use a Simon Says color theme suited to the app's theme to work in a nice easter egg. Lots of cool things with this!

Okay, I kinda really dig this idea - "pianistic echoes" like ripples in a pond.