It's not obligatory, but the inclusion of Rosalina is an open door to a 5th Zelda rep. And I think we should avoid another incarnation of Link/Zelda/Ganondorf, as the franchise have other things to offer.
Let's start with Impa: I highly doubt she will take Sheik's place. Sheik is Zelda, popular both in OoT and SSB. OoT Impa can fight, for sure, but I don't think she is as popular, and SS Impa have weird proportions, and I don't think it could go along Sheik's moveset.
About Tetra: She has some fighting potential, for sure, but her weapon is a dagger, and we know Sakurai don't like daggers. She could have some weapons like the grappling hook but she have really short limbs, and it won't help her in a moveset's range.
Vaati: Althought Vaati made 3 appearances, I don't think he is a likely newcommer. He may have abilities, but I don't see him having a created moveset that could bring originality. Also I don't like him
Ghirahim: Althought I like him, I also don't see him bringin in originality in smash. Except his tongue
Yuga: Not enough walls
This leads, to me, to only two likely newcommers: Midna (with or without Wolf Link), and Tingle.
Tingle: A recurring character, loved and hated by many people, that can bring a quite unpredictable moveset.
Midna: An original character, by look and behavior, that have manyshown magic abilities. She also comes from Twilight Princess, like Link and Zelda.
But there is counter arguments, like Midna's being "old" now.