Yes, Impa, Tingle, Tetra, etc may have unique movesets, but would anyone outside of a specific fanbase really care for them?
No, they wouldn't. None of those characters have appeared in a new game since Twilight Princess, which came over over a decade ago. Those games are more footnotes in the series and of video games in that year.
Their time has passed with the release of the hugely massively popular games that transcend the era they came out in like what Breath of the Wild has become.
The focus on Zelda seems to be squarely on Breath of the Wild going forward. Like I said, that's the first Zelda game that captured the eyes of mass video game enthusiast audiences since Ocarina of Time.
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As for Impa... she was shown as the traditional older sage role, not as a fighter (like in Ocarina of Time). So while I won't totally rule Impa out for the time being in Smash 5, I don't see how she's much of a candidate going forward. Hyrule Warriors I think barely keeps her in the conversation, maybe as a late add-on Sheik clone.
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I think the real question is what is up with Link and Zelda going forward. Link in Smash 4 is probably the best designed Link in any Smash game (sans maybe the Link in the Japanese version of Smash 64), and certainly the one with the largest toolkit. Zelda remains just as terribly designed and terribly weak as ever.
If Zelda got a total reboot with her moveset in Smash 5 based upon her Breath of the Wild appearance, most would be pleased.
Lastly going back to Link, I wouldn't mind Breath of the Wild Link being added as a newcomer per say. I think there's tons of material from that game in regards to his kit that you could easily transfer over to Smash. My only question when that is done in this hypothetical situation is what do you brand him as? Young Link? Teen Link? Wild Link?
Given that we got "Toon Link" as Wind Waker's Link's name (which I think has aged well in regards to a name), I would not rule out any such name in such a scenario.