I get the feeling that Mewtwo might be NOT the only DLC character.
Why? Let's take a look at the character data from Super Smash Bros Brawl.
We have the used characters there, sure,but we also have empty data of unused characters. Among them are:
*Dr. Mario
*Mewtwo
*Roy
*Dixie Kong
*Toon Zelda/Toon Shiek
*Whoever the hell pra_mai was supossed to be
As you can see, 3 of them are actually from Super Smash Bros Melee and are NOT leftovers (otherwise, we would have had empty character data from Pichu, but we don't).
And as if that's not enough, Dr. Mario actually ended up being PLAYABLE in Super Smash Bros for 3DS/Wii U, with Mewtwo on his way as a DLC.
Basically, Nintendo tries to do, what they couldn't do in Brawl. So naturally, in order to close the circle, they also finally make Roy playable as a DLC.
At least, that would be the most plausible thing, considering the empty character data of the unused Melee characters in Brawl.