While I definitely see the talking sword voice as Fi, Nintendo is a big fan of opting to create another, separate entity to stand in for something that should just be the same thing. If the voice of the sword given were ever given physical form, I'd be willing to bet it takes on a separate form treated as distinct from Fi with no indication where Fi is or what happened to her. Gives me Interlopers/Lorule/Dark World/People who sealed up Majora vibes.I mean, Fi is already in botw and not as a vague reference. She's the voice of the sword that Zelda hears. There is no other sword spirit associated with the master sword and they even use the same sound effect from ss when she calls out to Zelda (and again to Link at the end of the Master Trials). She's already present, albeit mutedly, but there's nothing vague about it. It wouldn't be strange if her voice appeared again in totk at the bare minimum.