You know, I've always liked Impa and have wanted her in Smash. But never at the expense of Sheik. Sure, there is some plausible possibility she can replace Sheik, but Sheik is a veteran, a character Sakurai is not just going to cut out in favor of someone else.
That and what pizzadudemanguy said. Sheik is generally a Special Move for Zelda, just as Samus's Transformation and the Pokemon change. You're keeping the concept of the Princess of Hyrule, Samus Aran, and the Pokemon Trainer intact. But having Zelda switch into Impa is something that makes no sense to me, for various reasons specified in pizzadudeman's post. They're DIFFERENT characters in one. Sheik is Zelda, ZSS is Samus, and the three Pokemon are the Trainer's, which is why they call him the Pokemon Trainer.
As her own character, it'd seem pretty redundant. Sure, she can be made completely unique, but we already have Sheik, which just makes Impa feel like an unnecessary add-on. The fact is Sakurai is trying to bring back all of Brawl's characters. So Impa replacing Sheik at this point wouldn't make much sense.