Cool concept! ¦3
I like clean visuals, bouncy effects, and outlines. You can experiment with making all the textures high-resolution, but rendering the game in low resolution (e.g. rendering to a small texture, and then putting it on the screen).
Maybe having them dance to the music beats would make it more immersive, as currently it is a bit painful to press the button at completely random time intervals. But I am biased here, as a long-time rhythm games player ¦3