No, separating the two can do more than that. As I said above, the problem with the Shelda design is that their attributes against each other which simply doesn't work. There's also still the issue of Zelda's moves being so awful in a game where speed and mobility are king. I almost wonder if the reason Zelda is so weak is because giving her too many options would've made Shelda broken by having too many total options.
CAN is the operative word. Doesn't mean it
will.
Even if Sakurai separates the two, there is absolutely no guarantee that they will be balanced differently than before. And even if they ARE balanced differently this time, there's no guarantee that the changes will be
better. Change is not always a good thing, as Ganondorf shows.
And that's not factoring in the inevitable change in the engine that ended up crippling quite a few characters from the transition from Melee to Brawl like Captain Falcon. There's no guarantee that the change wouldn't cripple Zelda further regardless if she's still a transformer or not.
I really hate to sound pessimistic like Habanero usually is, but it's justified here. Removing the aspect altogether is not going to be doing any clear favors.
And saying Impa on Sheik is like Waluigi on Luigi is not necessarily true. You're forgetting how art styles can change proportions. Case in point, Wind Waker Ganondorf was never fat once you realized that the guys in Wind Waker were deliberately drawn top heavy as seen below.
Except that the body proportion differences between Twilight Princess and Skyward Sword are near non-existent, so the Wind Waker example does not logically apply.
Impa's
really just a lanky, skinny chick, even when compared to Zelda. And since Zelda
is Sheik....
You get where I'm going with this, right?
Aside from that, I mentioned a model swap that Diddy posted where Sheik was replaced by Impa. I'm just going to say that Longchu looks less jarring in comparison and leave it at that.
Both may be true in another way. I highly doubt all the concept art we saw in Hyrule Historia was all the concept art that was drawn up during development. It's possible that there was an initial Sheik design for Twilight Princess that never fell into the hands of the public. The design we see in Brawl may have been a revision of this initial design. Still, this is speculating on something we may never know the exact answer to.
Actually, there's a third source; Eiji Aonuma.
He stated (before the DOJO!! reveal of Sheik, actually) that he submitted initial designs for Link, Sheik, and Ganondorf, then had to tweak them a bit to make them fit the Smash environment more.
This could very well prove your hypothesis that there was a Sheik design before the one in Hyrule Historia, though it sounds more to me that the TP Sheik design was specifically for Brawl the whole time. (Though that's just my take.)