I make visualizations of full symphonies and ensembles: https://www.youtube.com/watch?v=07Y3qC6rT2E
a major issue right now is note on/off priority. that is: new notes are currently being rendered UNDERNEATH notes that have already been activated. I think that this should be flipped around, such that if a note has been activated for a long time, new notes should appear on top of that one.
As you can see, synthesia layers new notes on top. I think this would be a very useful change for seemusicpiano and would improve the experience of multiple channel users vastly.