Yeah DJ Hero would be insanely hard to do, the machine vision part would be tricker for all the reasons you suggested. Simulating the scratch dynamics from the peripheral would be tricky too since it's not just a binary switch as far as the game goes but has certain tolerances which you'd need to calibrate. Effects dial you could ignore but I guess it would be easy enough to send a signal emulating that.

Agree that Beatmania would be amazing as a challenge and could build on the Jon Bot Jovi design!