Giga Bowser occurs when Bowser utilizes the star rod on the Giga Platform: a special stage built specially for the purpose of being able to crush Mario. Final boss in the game, only beatable with Peach Beam, insanely easy either way. It's where the concept was ninja'd from.
R.O.B may be a nice addition... I'm sure Paul would like him, seeing as he was his main in DS.