I haven't played that much Zelda, so I'd like to ask: How important is Impa in most of her appearances? I know she shows up a lot, but how often is she actually important and not just the exposition lady? I think she was pretty important in Skyward Sword, was part of the ensemble cast of Hyrule Warriors and pitched as part of a trio with Link and Zelda in Age of Calamity, and that's the extent of my knowledge. Was there anything else, or was she just a bit character in her other appearances?
Yeah well, that's the thing. Those are her bigger roles, but Skyward Sword especially is her biggest role. Story wise she is quite important in both Warriors games too. Whenever there's a 3rd most important character that takes the focus after Link and Zelda, it's Impa. She's just always around and thus a more safe bet for future inclusions.
But seeing as the Sheikah theme of Breath of the Wild set quite the tone , I do expect for the Sheikah to play a future prominent role in Zelda. And that leaves a lot of room for Impa to establish herself. BotW featured an older Impa, therefore TotK did the same, but Echoes of Wisdom also had an elderly Impa who was actually capable of battle as seen in a cutscene. I think that sort of counts?
It's not much, but the roles we have are prominent enough to warrant her inclusion. Skyward Sword HD is a Switch release, Hyrule Warriors was on 3 systems (Wii U, 3DS and Switch) and its sequel was heavily marketed as an actual "sequel" to BotW and did amazing sales wise. Impa was also hugely popular in Age of Calamity especially, because of the way she fights mostly but also her personality and design.
It's enough for me to consider her as the most viable character for now. She's easily realisable with how the Sheikah and Yiga fight in BotW and TotK, will always be around in the series, and at the very least would make for a good semi clone character of Sheik. Which might leave room for another newcomer maybe ? Since that's basically all we got since Brawl. New clone additions.
Might just be enough if the main cast (which is the highest priority always) is well represented. Ganondorf having a move set rework as well as Link and Zelda, with Sheik and Toon Link remaining mostly the same and then having a new face with Impa might just be "enough" to keep Zelda fresh in Smash for another generation. It KINDA was the same approach Ultimate took you know. Revamp Link and Ganondorf, add another clone by Young Link (who was already planned due to Everyone is Here anyway) and honestly... it wasn't much but it was better than Smash 4.
I see very little other potential candidates really. Skull Kid is a cute idea, but what about the future of this character ? It's not like the inclusion of Sheik in Smash allowed for future roles, in fact the role she had in Twilight Princess was entirely scrapped. It's only Ganondorf who had any lasting impact since OOT / Melee. And even he disappeared for more than 15 years till Tears of the Kingdom.
Impa is therefore the perfect choice as she will remain around. Same with Link, Zelda, Ganon(dorf) and younger / smaller versions of Link.
This is why I'll continue to defend Impa. Nobody else makes sense beyond the argument "this character was cool this one time". Take into account that she has exact the same amount of popularity as the most popular picks as Skull Kid and Midna and I feel she's literally the only pick.