I know this is a bit off topic to what you all are going on about, but I just thought of an interesting theory, that is if anyone hasn't already come up with it yet.
Its kinda clear now that they do intend on making these specially animated clips for newcomers, and usually in these clips they include other related characters, in this case Rosalina had Mario, DK, Bowser, Peach, Luigi, and Kirby appear. Do you think that the announced Peach and Luigi instead of other newcomers just because they were going to make this clip for Rosalina?
Lets say that we are getting a new pokemon character (or Mewtwo), wouldn't it be reasonable to say that if a new pokemon were to get its own animation clip, it would require pokemon characters to appear in the clip as well? As of right now we only have Pikachu. So if the next set of returning characters are Pokemon characters we could kinda predict that the next newcomer would most likely be a Pokemon reveal. Again they could do this with any series besides Pokemon.
Basically what I'm saying is, they can't make an animated clip with characters they haven't shown off yet (besides the newcomer), so keep that in mind when they announce returning characters, because for every returning character there is, that's more options for Sakurai to use when he makes the next newcomer clip.