I agree with Fiktion that Toon Zelda is more likely than a lot of you guys think. The Zelda series has gotten a ton of new entries since Brawl and is definitely deserving of an additional character. Regardless of your position on "clones," Toon Zelda/Tetra is easily the most significant Zelda character who has yet to be playable. Phantom Hourglass, Spirit Tracks, and Wind Waker HD have all released since the Brawl planning period, and Toon Zelda/Tetra is included in all three. Plus, she was planned for Brawl. At the very least, that tells us Sakurai considers her a worthy Smash Bros. character.
I can picture the reveal trailer: evoking the spirit of Super Mario Sunshine, Toon Link, Mario, and Kirby are lounging on a beach when a pirate ship pulls into view; the camera snaps in on Tetra who watches the three from afar with a telescope. "Tetra sails into battle!" Cue her gameplay trailer. Halfway into the trailer, there's another CG scene: Tetra approaches Toon Link, Mario, and Kirby on the beach, and everyone is confused. Tetra transforms into Toon Zelda, Toon Link is frozen in shock, Mario and Kirby stare at Toon Link, and then Kirby offers a cocktail to Toon Link. Cue a short bit of Toon Zelda gameplay. At the end of the trailer, Toon Link and Toon Zelda walk away together towards the sunset.