The Zelda roster is indeed weird, but outside of a newcomer, it's pretty decently represented in Ultimate. I am a strong advocate for Impa as a newcomer, and have been so ever since Skyward Sword was all hype and recent. And then of course Hyrule Warriors. I still think Impa would've made a great newcomer based of either HW or SS (Skyward Sword Impa would've probably been a Echo though).
But outside of that, they made sure each Link plays quite different from the other. Mostly changing Link and Young Link around, and keeping Toon Link sort of the same. Smart approach, because it worked. It's also great all Zelda characters take designs from different games / incarnations, though I do often wish Zelda was also based around Breath of the Wild (she is probably the most powerful, that's why). They finally changed Ganondorf around, and not only gave him a sword for his most bland and out of place attacks, but also made his playstyle work... at least.
I still would've loved Impa, and I think if Ultimate took the Smash 4 approach, a BotW Champion would be a given. Shame neither happened. Skull Kid also had great support, but still, I do think either Impa or Ganon would make better choices. Ganon would round out the Zelda roster completely because he's the main villain, and around both longer and more often than Ganondorf. Impa in the same way, yet both Impa and Ganon are sometimes just.. not workable for Smash because ; Ganon is either too gigantic or not even a complete living or conscious entity (Calamity Ganon in BotW) and Impa hasn't appeared as a young Sheikah warrior since Hyrule Warriors, and before Skyward Sword and is often an elder lady incapable of combat (my biggest gripe about BotW- as well as Impa not appearing as her younger self in cutscenes).
Just adding Impa would've been a big boon if she was just included in Smash 4, or now. Which would've been very reasonable given her relevance and popularity. But right now, not even adding Impa would be enough. At least, if she's not a unique fighter. However, it's very likely that the lesser relevant small Link will be cut if there's ever gonna be cuts in future Smash rosters, or both get combined into a more "Classic Link" taking inspiration for the upcoming Link's Awakening for Switch. Sheik could even be downrightly replaced by Impa with a similar enough moveset, or cut altogheter.
It's really hard to estimate a future for Zelda in Smash, because it's likely the next time a new Smash comes around the Zelda cast will be tweaked again to match the most recent Zelda game of that given time. And we don't know what's coming beyond Link's Awakening.
I still wanna see Impa and Ganon, being the most classic characters of Zelda who aren't entirely present yet (Ganon is a boss and Final Smash of course) and are likely to be mainstays in the future installations, but nothing is really sure. I do think Impa would make a lot of sense, because it's likely the Sheikah themes of technological inovation is gonna remain an element in Zelda games. The Sheikah are now way more established as a race, and Impa has always been their leader, no exceptions.
Anyways, that's my 2 cents on this.