Honestly, I don't know why they don't use costume choices to their advantage. I like to reference
what Injustice did with costume DLC when talking about character additions in Smash.
In case you don't feel like clicking that link, they gave alternate costumes different voice work when they represented other characters. John Stewart and Hal Jordan are different characters even though they're both Green Lantern, so when you selected John Stewart, you got John Stewart's voice. And vice versa. Same went for the CW's Arrow and Cyborg Superman.
Think about the possibilities if Smash employed a similar setup. We could plug a bunch of different characters in with Fox's moveset, like Krystal, James or even Bill. Daisy could have a different voice from Peach. Roy could have a different voice from Marth. Etc. That way fans of minor characters get their favorites without taking up a roster spot.
I'm just afraid that appealing to as many fans as possible in the roster isn't very high on Sakurai's priority list. He focuses more on the actual gameplay and various modes and collectibles. Which isn't necessarily a bad thing, but still. If modders could make these visual changes so easily, it can't be
that hard for the developers themselves to do something similar.