I love it! Very impressed, and very pleased with playing this. I would play endless stages of this, why not? It reminds me of all the educational games my piano teacher had on her computer. This game is great for musicians!
Playing by ear is fun. The trick/challenge, to me, is figuring out the Game Boy controls. Once you expand with more stages you could keep things even fresher by changing up the controls for a whole new challenge! But you'd probably want a clear break in the narrative of the game - "the character is playing using a different instrument now", or something.