I understand the logic, but I don't really agree with. Toon Link could have plenty of moves specific to him: an Up-B involving the grappling hook, controlling Wind with the wind waker, gliding with the Deku Leaf, the skull hammer for a strong finisher, and this only with the Wind Waker items. We can add the bomb-chu from Phantom Hourglass (althought it's not specific to this game) or the gust jar from Minish Cap, for example.
Same can apply to Zelda. The adult Zelda can have OoT and TP abilities, while Toon Zelda can have Toon abilities, including the phantom (even thought there is not that much abilities, I agree. Zelda already have to borrow Link's spell from OoT, but it fits her well).
Now, the thing is. We stated that though the Smash Bros games, Mario, Pit & Bowser had moveset changes. But, for Mario, he retained his Mario tornado as a down aerial, Pit kept a reflector as his down B and got something new instead of a blend of his down-B and neutral aerial, and Bowser seems to be essentially visual changes, as he still have his B moves. I agree Zelda got her first playable appearance (not counting the CDI games
) but it would be a major change to her gameplay. Can't say it can't happen just because it never happened before, but seeing how Toon Link didn't change despite being one of the most obvious clone, I don't really see why Zelda would get the favor. It's quite hard to change movesets drastically when people get used to them.
I know, the phantom would only change one move, but it become a whole different way of playing with the character. If you just summon it, the only help it can be is keeping people away and protecting Zelda. But it's clearly not an helpful thing when it comes to land a hit with Zelda's precise style. And if he keep people away, what to you do? Spam Din's fire? It can't really distract them either, as the target is still Zelda. And suppose you are trying to hit the opponent, but suddenly, the phantom comes and slash him and send him away. It would be just problematic. I just can't see how can be really helpfull.
If Zelda can switch to the phantom, it's not a great idea either. It would require a whole moveset, and how does Zelda work? Becomes an AI? And then, which level of difficulty? If Zelda dies, you are stuck with the phantom? There would have been more emphasis on this, so it can't be this solution, I think.
(everything I say above about the phantom is, obviously, a point of view, and some theories about how it could work).
It would be better if Toon Zelda have a moveset of her own, and switch to the Phantom (like Zelda/Sheik switch). We could have a new Zelda with new moves (because I think Zelda's moveset can't really change and become a quick but less powerful moveset), and have the phantom being a new heavy weight character.
Also, about Toon Zelda (and Tetra), there's the problem of the proportions. Tetra can't really work as a character as she doesn't have magic abilities, and can't use a dagger as Sakurai is against daggers. Toon Zelda, on the other hand, can use magic for extended range (like TP Zelda, actually). And phantom, obviously, do have some range.
I'm more toward conserving movesets. The only exception to me are Toon Link, as there were potential, and Ganondorf, because how the Great King of Evil can copy a futuristic pilot and bounty hunter?
This is quite wrong either, as Mario's moveset was decided way before Super Mario Sunshine. But again, it was not a big change, and he kept the Mario tornado as a down aerial. Zelda can't keep her transform as a down aerial
But she must keep her down B, as she is the only own able to transform without final smashes (Giga Bowser or even Samus), and there's no downside to this (Squirtle, Ivysaur and Charizard have a stamina system)