I know that. It'd be stupid to tell everyone the roster. That's why we have slow trickles of info each day.
What I meant is that Sakurai doesn't want wacky costumes that detracts a character's image. Let's take Link for an example. Link's tunics come in different colors and Link has worn different variations of it like in Ocarina of Time, Hyrule Warriors, etc. along with a "starting" outfit like in Twilight Princess, Skyward Sword, and the upcoming Wii U Zelda game. Those costumes and palette swaps would be okay, but having Link dress up as Legolas, a Green Lantern, Luigi, or Gregor from Fire Emblem: Awakening would seem out of place. Hell, people might even choose to dress as the other characters instead and that would take away the charm that is Link.
Oh, people want Fierce Deity Link! A costume based on that would be okay, but there's also potential for FDL to be a wicked character or a Final Smash for Young Link.
Lucina and Marth. Lucina was planned as a costume for Marth, but doing so would cement the idea that Lucina is a nobody and just a female Marth. That takes away from who Lucina is despite it being something Lucina wanted in Awakening, so she could be a great leader like Marth and the whole disguise thing. Marth is his own person; Lucina is her own person.
Luigi's a clown, but does he need to wear a clown suit? Now, if said suit actually becomes a power-up in a future Mario game, then sure, but for now, maybe not.