Mewtwo
Likelihood: 100%
Immensely iconic, and has a large resume of video-game appearances. Large fanbase (at least in America). Large backlash to his cutting in Brawl. Negligible hatebase. Game Freak has been charging up Mewtwo hype for years now, with videogame events and card game specials. This has culminated in the reveal of a new form for Mewtwo. Though it has garnered a mixed reaction, it has undoubtedly made Mewtwo a more current subject. Smash games have a tendency to add Pokemon characters that are relevant to the current generation, and Mewtwo now fits that bill just as well as he fits the bill of being iconic for Pokemon in general.
Furthermore, the act of adding Mewtwo, in and of itself, is a tempting one. Sakurai is in a position where every choice he makes is scrutinized by fans, and Mewtwo would be a definite crowd-pleaser. Mewtwo would also fill the role of a surprise addition, no matter how much he is predicted, because the act of adding in a cut character is itself surprising.
Sakurai has admitted to "thinking" about the addition of Awakened Mewtwo, in a situation where he could have simply remained hidden and silent.
Want: 100%
Yeah, I'm a Mewtwo fanboy.