Hm... maybe you could make it so he's usually wearing casual clothes and when he gets a powerup his clothes change to match what he got? Like if he gets a violin, he ends up in a tuxedo. He gets a boombox or whatever, he ends up dressed like a gangsta.

I have a basic design of what he looks like. Maybe I should plan the power-ups first before designing the character then.

Maybe something like this:
Violin- Slow Time
Boombox- Breakdance attack
Trumpet- Loud sound wave that acts as an attack
Drums- Smack opponents around with some drumsticks
Ukulele- um... get on a surfboard and hit em with a giant wave? XD
Flute- something with a samurai?
Clarinet- Snake Charmer
Guitar- sound wave in all directions (like that one assist trophy in brawl)
I'll list some more when I think of them

I like your ideas. I appreciate it.

how about something like a concert with zombies as audience? idk how to incorporate that idea into your game. just something that came to my mind