Then why would Tingle be bigger than Impa previously? Because all Tingle really had where spin-off titles. He also is notoriously unpopular with people, one of the very few characters I can think of with a equal strong hate base as a fan base. It kept him out of Twilight Princess for example.
Also, as of now, Smash has represented the GameCube era of Zelda twice now by having the character cast be based on Twilight Princess and Wind Waker only. Those are the only "important" games in the Zelda lore that Impa missed out on, and then again, she's still spoken off in TP in the Hidden Village and has a ancestor with Impaz, and there's a cameo of her in Wind Waker's Hyrule Castle camber where the Master Sword sleeps.
Anyway, to me it's logical and clear that Impa should be the next Zelda character. But I can see a BOTW Champion getting lucky and get in before her, or with a original moveset and Impa is either a clone, semi-clone, DLC character, or skin for Sheik. In which case I also wouldn't complain.
I think it's likely there'll be a Skyward Sword HD remake soon tho, maybe with additional content or more open world exploration. That would make Impa pretty damn relevant again.
Tingle got his own games. Impa didn't. That alone made him an actual bigger character. He's literally the main character in games. Impa has never been more than a secondary character at best, and technically just a small piece of each storyline till OOT slightly upped her role, and it wasn't even a major role. She has had small roles like that, where she's notable in her own right.
There's nothing to suggest we're getting a SS remake anyway. A key thing for it was the controls, and that takes a lot of work. It might be actually pretty difficult to capture the same motion control feel. It's quite possible it could be skipped remake-wise due to the controls. Unless they want to completely revamp it with more traditional controls, but that would also make a lot of veteran fans upset, so...
That said, Impa feels like a potential contender, but the thing she still lacks is a consistent design, and I don't mean artstyle, but the fact she has tons of canonical body shapes with none sticking out as her key one. HW isn't really all that important to Smash as is, with zero influence on Smash 4, despite being clearly worked on during the same development period(making it the prime time for content, but to be fair, much of the specific content is Tecmo's doing more than Nintendo's, so that could be part of it. Not a matter of licensing in the same way as making sure Nintendo wants those things to show up in Smash, since it's not something they mainly made. One of those complicated issues. That or Sakurai just wasn't interested. He clearly has shown more interest in Tingle's games too, which could mean multiple things). I would love some content from it, but it's pretty unlikely beyond maybe a costume reference at best. If Stickers were to come back, that's a different story. Trophies, on the other hand, have a lot more important characters to keep in mind of. Including main characters from the regular games, ones from the current games, etc.
As for the Sheik thing, Impa shouldn't be a skin for her. Honestly, she's too unique and it feels like a jab at Impa fans. Maybe a small reference like Smash 4 seemed to do with one of the costumes. I mean, even the Daisy costume for Peach is still more a reference than the actual character, with nothing but a literal palette swap(though ironically Daisy was that at first, but evolved as we all know, to the point that she looked beyond different come the Gamecube era at the very least).
I'm not sure we have a good chance for any Zelda newcomers. Moblin Ganon seems unlikely for multiple factors(no longer showing up in recent games, technically is a Final Smash since Ganon is the same transformation for Ganondorf in multiple Zelda games, albeit with differences depending the game, and then there's the case of whether they feel he adds something absolutely required with Ganondorf already there. Keep in mind that Ganondorf barely made it in, and there's nothing to suggest he would've made it in outside of a clone. Sakurai was very clear on the fact he was a lucky addition and he had no plans to include him otherwise. I mean, he could've still showed up in Brawl, but that's a pure guess and nothing more).
Regardless, I hope we get someone new. I'd love Moblin Ganon or Tingle the most, but Impa, while my 3rd choice, is still a good option. Not sure how they choose one Champion over the other though, so it might hurt their chances too.