If he had plans of seperating Zelda and Sheik, and giving Zelda the Phantom right away I doubt he would state that outrightely. I mean, the Phantom got shown with Zelda. Seems like if it is indeed a Down B, Sakurai already planned on giving Zelda another mechanic besides transformation (which in fact, is with Brawl's cast not as unique anymore as it was in Melee). I still feel that Zelda being revealed after Rosalina, who has Luma as a sort of 'summon' mechanic and her showing quite some alikeness to Zelda increases the chance of Zelda also getting a summon move on her own, albeit working much differently from the Lumas.
But really, let's see what happens. Impa would make a great newcomer, and could in a way be a spiritual follow up to Sheik. Sharing some common atributes and whatnot. Doubt many people (outside of Smashboards) would be that upset with Impa getting in if Sheik doesn't.