In fact, Wii Music could be an example of a more realistic free-form internal motivation challenge system. When people first start playing with musical sounds (remember, this game has been compared to a "My first music game" by Miyamoto), they don't follow notes or anything. They just hum whatever sounds nice, or tap out beats that sound sorta okay. To this day when I'm bored and fidgety I'll slap my thighs to some random rhythm that I develop on the spot without any guides.
I still feel that Wii Music misses several of the complex motivations and versatility that makes "musical play" captivating, but I'm not eager to accept today's "follow these explicit instructions" games as the be-all-and-end-all of music games. I have too much Mario Paint in me.