Referring to your first point: Touche. Zelda did bring something different than other transforming characters, and removing it would be an unfortunate loss in the uniqueness department, but like I've said - it's happened before.
I think my point still stands.
But referring to your second point, I do not believe that what I am suggesting is anything like asking to separate the climbers:
Neither Nana nor Popo could bring anything unique independent of each other. Having separate slots for both Nana and Popo would be ludicrous, as they are essentially the exact same character.
Having Impa replace Sheik as her own separate character is simply taking two unique movesets that could theoretically work independent of each other, and giving each their own slot. The only quirk is that one of the characters would then be re-purposed as another.
Honestly, your point there was unfortunately a weak one. I'm not suggesting anything nearly as foolish as having two identical characters as separate slots on the roster, to be frank. Quite the opposite, really.
I still hear what you are saying, and you've brought up many good points, but this specific argument is not helping your case.
Well, if I have to play on words, Zelda is still the only character that transform (the Pokemon trainer send differents pokémons, Samus change clothes).
My second point stems from the first one: the Zelda/Sheik duo is as unique as the Ice Climbers. It's obviously stupid to separate Popo and Nana, but removing their uniqueness is as foolish as removing the Hyrulean Princess' uniqueness (although her playstyle is already unique alone). So why it would be ok to remove one's uniqueness but not the other?
Hmmm... care to elaborate? I'm interested as to why you feel this way
Well, first of all, Sheik comes from one of the most known and loved Zelda game: Ocarina of Time. Important enough to have a remake (and not a port like ALTTP on the GBA). This game is also really important to the timeline, as it's where the timeline split in 3. It's also the game that created the base of the 3D Zelda gameplay.
To me, OoT is really a pillar of the Zelda Franchise.
Now, back to Sheik. In OoT, she's really a memorable character, as mysterious as she is. You don't know why she help you, but you definetely see her as a friend. Discovering it's the Princess in disguise is really surprising.
About cameos, games like scribblenauts or Tekken Tag Tournament features Sheik, alongside Zelda, Link and Ganondorf.
Also, Sheik managed to return in Brawl, something people didn't expected because she wasn't in Twilight Princess. Yet, she came back. If Sakurai can take a concept art to bring back an "irrelevant" character for Brawl, why wouldn't he take the character back now that she is relevant again with OoT3D representation in SSB4?
To speak a bit about Impa, she seems to be wanted by people, not to say acknowledged, since Skyward Sword. Until this game, I guess only a few people wanted the fat one, or the old one. I do like her Skyward Sword style, but what can she bring as a separated character, compared to Zelda and her ability to transform?
Please don't say a new down-B