At this point I think it would be a cheap cop-out to simply include as many alternate character costumes as possible, it simply wouldn't do the characters justice outside of characters such as Wii Fit Trainer, Pokemon Trainer, Villager, etc. It would also muddle the purpose of making a fighting/party game based on Nintendo's All Stars. Alternate costumes such as Link's OoT outfit, Mario's sports/ Dr. outfit are fine but to use entirely different characters in my opinion is where lines would be crossed.
Now peoples expectations are going to soar above reasonable levels as they are going to expect separate characters/entities as costume such as Lucina, Lyn, Roy/ Marth, Dixie/ Diddy, Daisy, Roslina/ Peach, Chrom, Hector/ Ike, Claus/Lucas, etc.