The Sheik thing goes both ways. Probably, she was planned and drafted for Twilight Princess, but the eventual Smash model was made from that for Brawl. They might've been undecided if they where to include Sheik or not in the actual game. However, this is pure speculation. Hyrule Historica says it was for TP, another source says it was for Brawl.
Then again, there where no real replacements for Sheik available back in the day. Now we have Impa. Keep in mind that Zelda / Sheik was one of the more unique newcomers from Melee as well, and Twilight Princess was basically Ocarina of Time 2.0. And besides Midna, it really had no other potential characters to be included. This isn't the case with Skyward Sword. And it took a majorly different approach.
I still don't see how people keep claiming Smash Link is TP Link. He definitely looks MOST like a texture mix of TP and SS Link, but still different. The fact they have Skyloft as a stage, and pretty much replaced the Bridge of Eldin with Gerudo Valley as a stage, am yet to be convinced it's Twilight Princess Link. Am making conclusions on this whenever I see Zelda and Ganondorf. Who I think will be based on Skyward Sword and Ocarina of Time respectively. Maybe Ganondorf could borrow from both Twilight Princess and Ocarina of Time however.
Impa is still a strong contender however. Especially with the Villager's Up B looking like it's from Balloon Fight. I just can't imagine Tingle getting in. And if he did, he'd likely have a Balloon Up B recovery as well. I just don't see them doing it. Vaati is not prominent enough, and has pretty much faded in obscurity. Am doubting we'll see anything ALTTP2 related, besides maybe a Young Link version based on this game (which I wouldn't like, Toon Link or GTFO please).
Ghirahim is quite a contender, but if he's in, I have a hard time imagining Impa not being in alongside her. Imagine if Toon Link was actually cut, and so would Sheik. That'd leave the Zelda series with just 3 characters. And seeing we had 5 since Melee, I'm not seeing it happening. Ghirahim would add a Skyward Sword flavour to the game, as he's Skyward Sword specific. Impa has a lot more appearances to draw from. And on top of that, she's a reoccuring character, who was in the beginning of the series. She's kind of like, the Toad of the Zelda series.