I don't see the point in getting rid of an established character. Sheik is obviously favored by Sakurai anyway as he picked him/her as one of the first announced Melee characters (before Zelda), made her an original moveset and brought her back for Brawl even though her relevance had diminished. Given the recent remake of Ocarina also, Sheik has had some of his.her relevancy restored. Impa had a big enough role in Skyward Sword, but Ocarina is considered the most important Zelda game by Nintendo if their timeline is anything to go by.
Also, I'm sick of popping on this thread and seeing Impa being brought up. I respect the fact you want her, don't think Sheik should/is going to leave and now supports Epona as the new Zelda rep. (Horses are awesome.)
Well, Skyward Sword is or SHOULD BE equally as important to the time line to Zelda as OOT is / was. It's the very first game in the timeline, "here is where it all begins". And its just about as revolutionary for the series as OOT was, or at least, a point in which things finally changed around in the series.
You popping into Impa talk is unfortunate if you wanted to avoid it, but we had just 3 official Not Talking About Impa Weeks, so the moment of silence is no more.
Also, Sheik being revealed before Zelda is probably cause of shock value. People wouldn't suspect her, same thing happened with Brawl and Zero Suit Samus.
Thing with Sheik is though is that she was never really all that important anyways. Not enough to really warrant her spot in Brawl, but it was justified somehow by her planned inclusion (plus workable model) in Twilight Princess, more or less.
Her MELEE popularity also would've helped with this, but one cannot deny there are / have been much better candidates for Zelda for Brawl than Sheik. As her role isn't too great in the overall series of Zelda, or even Ocarina of Time. Besides, OOT gets enough love already with Zelda's spells and Ganondorf himself. Gdorf might even be all inspired on his OOT appearance, and you could say that he was much more important to OOT than both Zelda and Sheik anyway.
Impa had a lot of character development in SS, and quite an enormous role if you pay good attention in the end. It's no reason by itself, but considering Zelda / Impa could be a far more unique, improved concept over Zelda / Sheik, why shouldn't importance, relevance and overall role also be considered? It's only a better reason to actually do it.
Impa originally was the holder of the Triforce of Power, and has been around forever, with a recent revival with a lot of character development in the 25 year anniversary game... She even might become a staple to the series from this point.
Still not everything I have to say but I don't wanna write a big essay on this too much now. But I feel Impa has more than what it takes to make it as a character.