I want to start off by saying I really enjoy Heart Attack. I think the concept is super novel, and the character art is especially charming. I like how the characters seem to mirror each other in some ways, and having four units for you and four enemy units feels intentional. The motions feel quite snappy, especially for the character that runs back and forth. And I think with your goal to have this be a prototyping kit for rhythm and music games, there is room for expansion for additional units on both sides. It's also a very nice touch to have the tiles light up with the heartbeat. 

I like the generative path, but I feel as if you could do more with this. A single path feels very 'easy' and  there might be some additional strategy if you had more than one path leading to the heart! As well, having the play space grow larger could factor into the music if it had 3D spatialization. Then, panning around the space would reveal different soundscapes, and would add an extra layer to the musicality. It might also make it feel more 'imposing' as enemies advance, as the music would grow louder as they approached (though this may be difficult to manage with the number of audio voices, perhaps the number of units plus their distance could contribute to the overall volume.)

I really look forward to seeing if you add any other units, and what their contribution (ability wise, and musically) might be!