Seeing as Doc was planned for Brawl and was in Melee, I'm pretty sure that if there is going to be 2nd Mario again, it's Doc, not Papes.
Note, that's IF there's going to be one.
I mean, while Doc is not nearly as requested to return as Mewtwo and Roy (far from it, actually), he still WAS planned for Brawl. We know that Sakurai looks back at old concepts he couldn't quite do and characters he couldn't quite add.
As for the costume BS, it's not going to happen. Doc's either coming in with his differences intact, or he's not coming at all.
Best scenario: Dr. Mario doesn't appear on the CSS so the purists don't have any reason to complain, but while selecting Mario, before selecting him, you can toggle between the two by pressing the Start Button or it's equivlant, similar to how Sonic the Fighters HD handles Metal Sonic, Eggman, and Honey, characters not originally playable, and thus do not have spots on the CSS.
That way, Doc gets in, he's not taking up space on the CSS, he's got his gameplay differences, everyone wins.
EDIT: And to suggest that he will only get in if EVERYONE comes back is asinine. Young Link and Pichu were not planned for Brawl, Doc was. He's the only one of the three that could come back.