I've just submitted this for the 2018 Hackaday Prize musical instrument challenge. I've really enjoyed building this around the Teensy 3.6, and I hope others will build one too.

https://hackaday.io/project/161127-teensy-beats-shield
https://www.youtube.com/watch?v=ZFfMhkVe_6c

Source files
https://github.com/trailhead/teensy-beats