Well, that all depends. Its possible that a character they started working on before release just barely missed the cut, unless they started set character late and/or cut said character early, the character should be in the finishing stages of development, as opposed to Mewtwo who likely barely started. If anything, if there's a character they didn't quite finish, they may release said characters through a patch instead. Keep in mind, Sakurai said that no paid DLC is planned. However, they might've not quite finished a character they're still planning to release before Mewtwo.
I'm not saying its going to happen, but its a definite possibility we need to consider.