Except Jigglypuff occasionally.
Also any of the cut Melee vets not named Mewtwo.
Ness in Melee to. If Mother 64 came to be, there'd be Lucas in Melee, not Ness. Said by Sakurai himself on the Japanese Melee Dojo.
Time will tell if I'm wrong, but I think Sheik is special since she IS tied to Zelda, who is a more immortal character. My guess is that, unless they see a pressing need to change or eliminate Zelda's ability to turn into Sheik in SSB, they won't. But maybe they DO have a reason or see a need to do so. They didn't in Brawl, so I'm unsure why that would be any different now.
Because Zelda is a seriously badly written character? And the 'implied playstyle' is just wrong. Soften foes up with Sheik, who has swift, weak but safe attacks. Finish off with Zelda, who has slow, occasionally strong, and unsafe attacks. It's a recipe for disastrous balance.
I think because Sheik is tied to Zelda, and clearly dependant of her inclusion at first to be considered, Sheik has a clear disadvantage. Sheik, like Zero Suit Samus, clearly isn't the 'main' character. Zelda and Samus are. Pokémon Trainer is different, all Pokémon are equals, and serve 'under' the Pokémon Trainer. Charizard might be different though, as he's popular as ****.
Sheik was also drafted upon for Twilight Princess. I can claim this with certainity, cause there where also Sheikah characters planned for Twilight Princess. For example... Old Man Impa:
As the 'Hidden Village' in TP had connections to the Royal Family. It isn't so farfetched one of the Sheikah there could've given Zelda training, as to become Sheik. Sakurai himself even stated on the Brawl Dojo ;
'Sheik doesn’t appear in The Legend of Zelda: Twilight Princess, but we based her model on a design that was drafted up during the development of that game.'
http://www.smashbros.com/wii/en_us/characters/sheik.html
So, knowlegde says, Sheik was planned for Twilight Princess. At least, probably still till Sakurai made his decisions of the roster in 2005. Take note he took 'recent' characters as Lucario and Ike also in consideration, so he for sure had inside information. Which again, seems to appear to be the case this time to, as Rosalina made it in.
I'll get you Sheik! Just you wait and see!