I figured this might come up and I just wanna make a distinction in how I judge these similarities. I'm thinking in terms of character hook.
I don't consider using a sword to be a "hook". Maybe back in Melee it was special when Marth did it, that's his thing. But nowadays when this is so commonplace these characters all have some other thing going on... and to your point yeah, this could theoretically be the case with characters using music as a thematic throughline. Because there are so many kinds of music, and performance styles. But "theme" is the key here, because this is a fresh new thing we've never done before and is more a general aesthetic or character-infused approach rather than something so general as a style of weapon. I feel that if some of these characters were added, there's a strong chance their core hook is "they are a performer / musician". That'd be the big initiative to having some of them here, especially Pauline.
Would it be a matter of shutting it down immediately because "we already have a music character"? Not really. But I'm trying to visualize Sakurai's pitch document, where he might say about... I'll use Pauline as an example, "she is a performer who attacks with music". A lot of them end up being simple couple sentence taglines like this, just a general concept to elaborate on. I have to imagine if Chorus Kids, or KK Slider were on the table their "elevator pitch" might be described in a similar way with one or two different details - add in "has a guitar" or "can attack in rhythm". But I think that baseline overlap would generally hurt their chances of being considered if Pauline was the first one that came to mind. Big what-if, of course.
I still think the most likely are the Splatoon idols, who I suppose after thinking about it some more are also the ones who stand on their own the easiest. Because you potentially have a team-up, duo thing going on and you're using some wacky ink weapons, so that stuff definitely serves as much of a "hook" as being performers does. I still think there's the possibility that "fights with dance choreography" is something that could clash between Squid Sisters / Off the Hook or Pauline. But there's admittedly enough going on here where it's easy to envision other directions that avoid that problem.