The next game should promote the Yiga Clan to the primary antagonist and have Impa lead an attack on them.
This alone has a far better theme for a spin-off than everything Tingle has ever done
Cool concept though! And yes I'd love to see something similar happen to this. The Sheikah-Yiga rivalry and all the other events before BOTW where the Sheikah where the dominant people of Hyrule really would make for a good game concept. It also made me realise that where Impa to have a bigger role in BOTW, as in Skyward Sword, she would probably take over the leading role of Link and Zelda. Especially since BOTW isn't all THAT story-heavy.
Anyway, there's a lot which they can do with Impa's character at this given point. What I'd like to see would be another sort of double role like she had in Skyward Sword. But spread across two games even maybe. The Sheikah can live for a long time through meditation it seems, it wouldn't be farfetched to have Impa in two different games, if the games make a clear connection towards each other. A prequel to BOTW could very easily establish this sort of role for Impa for example. Or what about the First Hero who fought against Demise's army on Hylia's side prior to Skyward Sword? Seeing as the map layout is quite different in Skyward Sword, they could just base a new game like BOTW on a map inspired mostly on Skyward Sword.
Impa would be there again as well, and probably even Ghirahim. The origin of Calamity Ganon can even be explained further by giving Demise a stronger backstory, as well as Impa's dedication to Hylia, or even the Sheikah race as a whole. In this time, there probably wouldn't be Yiga yet. But that could be saved for a sequel. They could establish the origin of the Goddess / Master Sword and Fi, Impa and the early Sheikah with their possible technology (think of what was present in the Lanayru Mines, and BOTW), origins of Ghirahim, Demise, and even Ganon(dorf) and Calamity Ganon, origins of how the first Kingdom of Hylia (predating Hyrule) got to be.. why the land was send to the sky and all... Seems like a solid game, on paper.
What am expecting is something totally new however... What exactly, I can't say. I do expect Zelda to continue down the road of BOTW, but trying to add more and more classic Zelda elements into it, as dungeon exploration. Which is all what was really missing in BOTW, as well as Young Impa of course. I do hope for more character growth with Impa in the franchise, yet after BOTW, I can't say what's gonna happen. Seems like they prefer to add new characters to add to the story instead of building up on the ones they have so.. I'd be pleased to see something else happening.
Another potential game I just started to think of is a Zelda based far more on stealth missions, but using a similar engine to BOTW. Nintendo has expressed how they wanted to use Sheik maybe for a future Zelda, and if Sheik where to be a main character, stealth would be her gimmick. And this would also allow for a easy role for Impa as tutor / instructor / mentor kind of role that's probably gonna be pretty damn major.
I would love for Impa to get in this game (specifically her Hyrule Warriors design). To be honest, I can't really be considered a "true" Impa fan since I never cared about her until Skyward Sword. I always found her Ocarina of Time design to be a bit lame. I have been on a huge Hyrule Warriors binge lately and Impa is my favorite character. Her design in that game is one of my favorite of all video game characters.
I've seen a lot of people suggest that she should be an echo of Sheik, but I feel she has a lot more to offer than that. Either of her movesets in Hyrule Warriors would stand out and offer something unique to Smash. Her greatsword would make her one of the few water elemental characters in the game and she has a very distinct style compared to Ike or Cloud. If she used her naginata, she would be the only polearm user in the game aside from Palutena (and Bandana Dee if he gets in). Worst case scenario, I would still be happy and accept her as an echo of Sheik.
I would also gladly accept Impa as a Echo of Sheik at this point. Not only because I just desperately want her in, but I originally also wanted Impa to replace Sheik and take over her moveset or at least playstyle. The new Final Smash is also very well fitting for Impa, and Sheik is probably gonna be Top Tier or at least viable once again, so that's a definite plus. There's alo enough Impa could do to make herself shine as her own character through being a Echo, explained this before.. yet am not 100% sure now if my ideas are a bit too ambitious.