In fairness, I could appreciate Impa as a playable character because to me she's one of the 'big 5' Zelda characters: Link, Zelda, the 'Demon King' (Demise/Ganondorf/Malladus), and then Impa and Tingle, as the two most re-occurring characters without Triforce pieces and in Impa's case, a fairly big character in Zelda since the beginning - though her relevance waned a bit in the Gamecube era.
Were she in, it would be great to see her as an amalgamation of her OoT and SS incarnations, with a red robe thrown in to reference to
her appearance in the original game and
her 'old' form in general.
That being said, I don't necessarily believe Impa has to take Sheik's moveset in any way. Currently it really does depend on whether or not they're going to go with Skyward Sword designs for the Zelda cast, TP-esque designs for the cast (a la the Wii U tech demo) or something else entirely, like making new incarnations of the characters for Smash (see: StarFox character designs in Brawl). My primary concern with Sheik's return at the moment is that it would be cripplingly out-of-character for Skyward Sword Zelda to transform into Sheik, being that she's a much younger, less-mature Zelda than those in OoT and TP and has completely different circumstances. I guess for me it really does come down to whether or not they can make Sheik 'fit' into the roster, because if they go the typical way of basing the Zelda character designs off the most recent generation of releases, then I wouldn't really appreciate Skyward Sword Zelda or Toon Zelda having a Sheik transformation.
But at the end of the day Sheik is a fan-favourite. Although I'm on the edge about her inclusion I don't doubt that the team will find a way for her to fit into the game and still feel consistent with everything else that's going on. Sheik and Impa don't
have to be competition for one another; it does seem to be an entirely fan-made conflict that probably won't have any standing on the roster.