I'm still waiting for a drum machine app that adds elements from Rockband. Have instructions for what to tap next come in just in time for you to play a beat. Then I could play the song Reggie made while getting deeper into the app. It doesn't have to be jewels falling from the sky like Rockband -- maybe a pulse emanating from the center of a cell that you need to tap when you reach the edge. Like this:
SlidesUp API
Shisharka Land
TopBark
YesGraph