Why is that shorty? Afraid that people actually like the idea? You guys don't even consider it before blasting it of all the time.
Impa is a very legitimate choice to fresh up the Zelda roster. Because like Mario, every important character is already in, with Sheik. Sheik being Zelda transformation is an OoT ONLY thing. And she got lucky in Brawl cause of that spare design.
Sheik was probably added in Melee to have a ninja archetype fighter. And it was a cool reference to OoT, at that time. Zelda WAS Sheik in OoT after all. But that's not the case in all other Zeldas. And seeing the trend they have been following with basing Zelda characters on their latest appearance, expect the same with them in Smash 4.
As said before, the spare unused Twilight Princess design kept Sheik in, cause she was. likely still planned for the game, and Zelda didn't had much potential to draw of from that game (could have been given the rapier, but just look at what they done to Ganon :/). Hence her being in wasn't TOO bad, yet disappointing. Especially with Mewtwo gone for a newer character.
Zelda didn't have much of a role in TP, but she did shown a LOT of character development in especially Skyward Sword. It's easily her most important role yet. As a standalone character even.
Now to Impa again. She might've been quite forgetable in other games, but that didn't stop Nintendo for giving her this role in SS. It's huge character development in a game series where its rare to have any. Exceptions being the characters we have in Smash, and one offs like Ghirahim and Midna. There is also Tingle.
The fact that she also was brought back to the series before Sheik also works with this arguement. I expect Impa to reappear in Zelda far more often now, thus growing ever more important. Hence why I think Nintendo will personaly tell Sakurai to have Impa over Sheik. The fact that she's also actually a fighter while Sheik is not helps enormously.
And as said by others, and now Ice Prince, I doubt people would care much if this happens. Cause: IT'S NOT LIKE IT DIDN'T HAPPEN BEFORE. Impa could have Sheik's style of fighting, separate Zelda, fresh up the roster and represent Skyward Sword. The pros imo outweigh the only con being Sheik is a veteran. But people hardly play her for being her, they play her cause she's a ninja. Impa could be a half monk, half ninja. She is both more unique and more important to the series. Her replacing Sheik is more logical than Lucario replacing Mewtwo.