Mipha and Daruk huh? Yeah they seem the most popular and unique as a concept for a Smash character if we'd have to pick from the BOTW Champions. Yet, I don't think they will have lasting influence on the Zelda franchise. Then again, neither did Sheik have a single role in a mainline Zelda game after Ocarina of Time and Ganondorf also hasn't appeared since the Twilight Princess remake. But they can somehow be excused as being veterans, and versions of Zelda and Ganon- who WILL reappear in some way or form.
And that's where Impa shines, cause Impa is also almost always guranteed to reappear. She appears as much as Ganon does himself as of late, which makes either Impa or a workable Ganon as the most steady 'core' characters to add as Zelda newcomers. Ganon being workable is highly debatable however. But Impa indeed lost a lot of base as a Smash newcomer because of her role in Breath of the Wild.
Oh man I remember how DISSAPOINTED I was when I saw BOTW Impa first. I spoiled myself just to see her, looking through all the early spoilers and stuff just because I needed to know how Impa was implemented. I was so dissapointed that I innitually didn't even wanna support Impa anymore, but am over that now.
If Sheik, Toon Link and Ganondorf return, the designs of the Zelda characters will be all over the place. In which case I think there will be a lot of alternative costumes, in order to gain some "lore canon" in the roster. I predict OOT, TP and maybe Hyrule Warriors designs for most of the cast of Zelda. And in that case, Impa would fit in fine with the rest.
Am still not sure if they would add in just ONE of the Champions in favor over 3 others, but I could see it happening with Daruk as he would provide a nice heavy weight concept of a newcomer, plus his personal ability would translate over very well and Gorons are a huge steady staple of the Zelda franchise, moreso than Zoras, Gerudos and Rito. That being said, if Impa is a semi-clone or downright last minute clone of Sheik, there's room for both her and Daruk, or another Champion (I say Urbosa).