Guitar Hero ("On Tour" ) for the DS added additional controler buttons to allow you to hold it like a guitar Hero guitar and use their buttons in the game. Was a really cool approach.

https://en.m.wikipedia.org/wiki/Guitar_Hero:_On_Tour#Gamepla...

Edit: a better picture of it in action, https://en.m.wikipedia.org/wiki/File:Guitar-hero-on-tour-ds-...