Legend of Zelda: Xylophone of Souls.
And you could use the Stylus to tap on all the bars! And maybe for customization you could change the colors of the bars to whatever color you want. (Is there an actual musical term for the Xylophone bars? I'm not sure).
It'd be kinda hard to carry around though.
But seriously, a violin would be so cool. Especially as a Wii game. It may be difficult to work out how exactly it would be played, though. Perhaps how fast you move the "bow" across the strings, it plays a different note? Or maybe the nunchuck buttons would serve as tuners?
A marimba or xylophone wouldn't be bad, but that'd be difficult to cart around. ^_^; Marimbas are heavy mothers. Perhaps a crockenspiel? =P
(disclaimer: I love the accordion to bits. But not in a Zelda game.)
*Eyes widen, and palms sweat* MUSIC!!! Yay! I love music threads. Anyway... Personally, there are a couple instruments that I would like in the series. I would enjoy a Marimba or Xylophone, but like its been said, they would be really heavy to lug around.
My friend made a fan fic of Nintendo games (well, more like a parody than fan fic). He made me Link, in essence, and my instrument was the marching snare drum. I think it would be cool to have to play complex rhythms and rudiments on the drum to act as songs.
But honestly, as long as it is cool and keeps the series fresh, I'll be happy with any musical addition to the series.
I'd like to see Link make an instrument. ^^ I think it should be wooden and cute, that makes a nice sound. You could collect items throughout the game that all come together at the end as an instrument that plays a key part in the plot. I would say something quite big...maybe a harp or some kind of guitar type thing. Or maybe a violin, because you use horse hair for it.