Wii Music didn't exactly land well with critics, even if it did okay in terms of sales. But a sequel could still be on the way, and hopefully Nintendo will have taken the feedback from the first game into account.
Shigeru Miyamoto told Techland the following in an interview:
"There's a lot of potential still in Wii Music, I think. Because we're going to change it up, a new interface is coming, and all of that."
When the subject was brought up whether Mario might be introduced in a possible sequel Miyamoto had the following to say:
"If we get more people to understand what's going on in the game concept, then he might appear in the game."
Sounds like they're working on something at least, but as always with Nintendo it's not certain to see the light of day if it isn't good enough.