If there's gonna be any Zelda newcomers, I think the one with the most potential, while simultaneously being a big character in the series, is Toon Zelda. The Spirit Tracks incarnation inside the Phantom.
View attachment 140750
The ability of a character's shell to break with excessive damage and become a weaker, but more agile fighter would bring something completely unique to Super Smash Bros. There's many incarnations of the Phantom that she can derive her abilities from. Such as Torch Phantoms, Warp Phantoms, and Wrecking Ball Phantoms.
It may seem highly unusual for Toon Zelda to not be a clone, but there's nothing preventing it, especially now that the differences between her and the BOTW version of Zelda are greatened.
It's the most iconic incarnation of the character and it was used in Hyrule Warriors. It would also make sense assuming they decide to use the newer incarnation of Toon Link from Spirit Tracks like they did with Adult Link.
I think it makes perfect sense for future Zelda newcomers to be other incarnations of the current characters, seeing as there aren't really any other characters available. I feel like this is an exception they can easily get away with as the other incarnations are completely different characters as well as the most iconic ones in line.
Like I said, this would simultaneously be the most popular/iconic as well as the most unique character available.