Here's what I think could be improved in Wii Music Plus:
- MotionPlus support: Some of the instruments didn't work in the original. 1:1 controls could give players more control in their performance and add more depth than just waggle.
- Music of greater quality: Even if they use MIDI they can make better sounding songs and musical notes.
- A professional mode: The biggest complain about the first game was that it was too easy and simple. Adding a recording studio of some sort with tons of features would give advanced players the option of creating their own songs, in a similar manner to the new Guitar Hero games.
Hell, you can create a song, play it, then send it to friends.
- More variety in music: The public domain songs were fine, but there was a limited number of popular and Nintendo songs. This should be fixed in the next game. Add more songs from all genres of music. Miyamoto once said that they wanted to add more than a hundred songs, and this would give him the chance to do so.