There's no Toon Sheik, and there never will be a Toon Sheik. Especially not in Smash.
Zelda might get revealed tomorrow. I'll tell you, it's 100% sure Sheik won't be featured in that update.
Impa might take a backrole in ALBW, but at least she's there. Can't be said about Sheik in any game that's not a remake. She didn't even had reason to be in ALBW, cause she wasn't even in ALTTP. She is becoming more and more of a staple. So be sure she'll be in the upcomming Zelda games with bigger roles like in Skyward Sword. She'll be in Smash one day, and Sheik will never be in a Zelda game again that isn't Ocarina of Time.
Impa > Tetra >>> Sheik >>>>>>>>>>>>>>>> Toon Sheik
No matter how much you like or dislike a character personally, it won't affect that character's chances unless you have a horde of people backing you up. Impa might have more roles than Sheik, but there's not any noteworthy demand for her. Sheik on the other hand has reasons enough to stay, being a Smash veteran already gives her a big edge over Impa. Brawl did make it clear that recency doesn't matter, and if it does, then remakes do matter, Pokémon Trainer having the Kanto starters goes to Sheik's favor with both arguments. Furthermore, Sheik has just as much reason to get in this time as she had in Brawl.
It's nice to really want something, but don't fool yourself with it, Sheik is much more likely to stay than a good chunk of other veterans, for exactly the same reasons as in Brawl, while Impa is an obscure character due to her minor role she has in by far the majority of her appearances and the fact that she doesn't have any noteworthy support behind her.
Not really expecting Sheik to be revealed alongside Zelda though, and even if Zelda has her SS design, I'd still say that Sheik's more likely to get an updated design to go along with it, again, like with Brawl.
That's my two cents on it. Dislike Sheik as you might, you gotta admit that you have the odds against you.