Majora's Mask got a remake in 2015 and Twilight Princess got one in 2016. Given the development of Smash Switch started in 2016, this is something to keep in mind.
In that case, I'd love Twili Midna and Skull Kid.
But we might just get Young Link back, and I wouldn't even be opposed to that.
Honestly... What if the Zelda situation got so dire and Sakurai really didn't know who to add anymore with all the requests for "Zelda newcomer" or "Zelda rep" (think "DK rep") that he'd just add Young Link and Toon Zelda as new characters and call it a day?
This is why am 10000% NOT picky when it comes to Impa, because I'd take a downright clone Impa over nothing honestly. Or at least, over Toon Zelda (Sheik is already there as Zelda 2). Clone Impa and clone Young Link in a weird way would already satisfy me honestly.
And yes, Young Link can easily be a clone and still represent Majora's Mask. The Final Smash as Fierce Deity Link could already work similar enough to the Triforce Slash anyway.
Ideal situation: Slight moveset overhauls for Zelda, Ganondorf, Toon Link and Sheik, new revamped Specials for Link (plus Final Smash maybe). Toon Link, Ganondorf and Sheik based on Hyrule Warriors, Link and Zelda based on BotW. And then Impa getting in as a full representation of Hyrule Warriors with either Naginata of Giant Blade.
But honestly.. I'd settle with Impa getting a few moves from Sheik. She could be based far more on Melee Sheik for example. Think the old Melee F Air for example, just forget about the Chain please.