I think "costume changes" that change the character would be dumb. Mario had a Wario outfit and Peach a Daisy outfit in Melee, but that was just their clothing. Having Fox magically change into a different person would just be strange.
(Note: I'm not even sure what triggered this, but here's my thoughts on characters...)
I don't really care if we get waaay more characters than Melee, simply because Melee had the 6 clones. 1 more original moveset would probably have added more depth to the game than all 6 clones did. In short, movesets > character selection screen spots. And in terms of movesets, there were only 20 total in Melee, and only 8 of them were new. In Brawl, there are already 22 confirmed movesets, and 10 of them are new. If we just got Luigi, Falcon, Ness, Sheik, Jigglypuff, Mewtwo, G&W and Marth back, plus Ganondorf and Falco with new movesets, and got no more new characters, that would bring the totals up to 32 total and 12 new, yet only 29 character selection screen spots, and only 7 so-called "new characters."
But, of course, I don't expect that to happen. Given that we have two and a half months until release, and likely won't see every character before release, that would be a very pessimistic prediction. I would not be disappointed with 32 character selection screen spots and 35 total movesets, though, even though some jackasses would say "OMG ONLY 7 MORE THAN MELEE? SAKURAI YOU LAZY ASSHOLE."
Of course, in my dreams, King Dedede, Dixie Kong, King K. Rool, Lucario, Ridley, Sonic and Mega Man will all make it in.