A music pin can really be just about anything you could think of...that's the good and the bad of it.
Sparky was just made up, but works really well for Metallica. For Aerosmith that had the Jacky character...pretty cool overall toy. The Gene head didn't work too well, but the concept was on point with balls coming out of his mouth.
The first time around, Slash's love for snakes's was a big inspiration, and you were basically assembling all the members of the band together.
I had an idea...don't know if it's good or not, but the setting/theme of the pin could be the 80's sunset strip in Hollywood. The band formed there, and there early days/music were heavily influenced by the strip.
You could get pretty off the wall with a music pin too. You could have like a battle of bands where they are "killing" other bands...like mortal combat meets guitar hero. The band members could form into a Voltron style machine. Or maybe go Bill and Ted's Excellent adventure route and they're saving the world with music.
It will be interesting to see what they have come up with.