This Week, Luke Plunkett from Kotaku says (in There May Be More Wii Music) It's easy to make fun of Wii Music, but remember, it still sold over two million units. Which probably explains why Nintendo are still talking publicly about the possibility of more of the things.

Exciting, no?

Wii Music: You Haven't Heard The Last Of It [ONM]

And hey_suburbia from WiiNintendo says (in Miyamoto talks about possible “Wii Music Plus”) Miyamoto said this to ONM: “I think it sold lower than our expectations but on the other hand it was a really unique game, I have to say. And also, even today, we are receiving several different offers from people in the field of music education, so as far as Wii Music is concerned, I think [...]

And CVG Online says (in Miyamoto drops 'Wii Music Plus' hints) Wii Music to see "future developments".
It may have failed to live up to expectations both critically and commercially, but that hasn't stopped Shigeru Miyamoto from insisting his Wii Music still has a future, and even hinting there may be a 'Wii Music Plus' on the cards.