It seems Sakurai mentioned alongside the new characters and stages, a new Master Ball Pokémon has been added into the game
Will try to find if it's true
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
This would make the most sense. Better yet, if it was the dog form of Zygarde, that could make for some good buildup towards Pokemon Z.
My money is on Volcanion to promote the upcoming movie.
This is also a good guess. If a legendary Pokemon is to be added, it would have to be one of these two. I wonder if Zygarde or Volcanion came across Sakurai's radar of possible DLC characters, but due to timing and also moveset, he thought it would be best to leave them as a pokeball item.