I'm not saying she won't be included. It's entirely possible that she will, if she becomes at least a semi-clone, but I have a hard time trying to find out how to make her different enough from Zelda to not be a clone, which makes me somewhat skeptical.
Toon Link didn't have to be entirely, or even notably different from Link to be included, did he? Toon Zelda was planned previously, even if just for fluff Sakurai obviously was fine giving her a semi-cloned moveset, so even if you have trouble picturing it, he doesn't, and that's what matters. It's not always all about being original and dissimilar. In fact oftentimes a character's chances increase
because they aren't original and because they are similar, despite what Sakurai has said. He's added clones (or semi-clones) in each game so far. Maybe he won't this time, but it's not like there's no precedent.
In the series, yes, but as of now, I think Tetra has a slight edge because of her importance in WWHD and Phantom Hourglass.
My fault, must have forgot about the remake at the time, however, I believe Tetra has more importance in WW than Zelda does from what I've seen in the Hyrule Historia and on the internet itself, but I will see once I get a Wii U.
But that character is still Toon Zelda, Tetra is just a facet of her. She's not the main form, and even if less original Toon Zelda still has the edge over Tetra due to overall larger impact to the series, plus the fact that she was planned, and yes, even being a relatively easy potential clone is a boon for her. Obviously there's a more than decent chance she might transform into Tetra, but we're not going to get the transformation without the default. We wouldn't get Sheik without getting Zelda.