Shouldn't this be part of a much wider topic subject, like "Official Mario discussion" or "Mario forms discussion"? I'm pretty sure don't two topics haven't been created yet, but that's beside the point.
I'm against any Mario form getting in as a different character. Should Sakurai do something similar to the costume idea, I could them getting in that way. I know the different forms have different attacks, but the thing is, their already part of Mario, like fire balls and the cape (even if it works differently). It's also kinda like how they gave Kirby different attacks from his many forms. At least Zero Suit Samus plays differently from Samus, where none of the Mario forms are that different enough for such at thing.