I think Ghirahim, Tingle, and Toon Zelda / Toon Shiek are the three worth mentioning. I don't really know how I feel about Toon Zelda / Toon Shiek. I think she has a definite chance of being included because she was chosen to be added to Brawl, at least at one point. I don't know how much water that holds, because she was cut, and we HAVE seen characters cut and never return, like the idea of Leaf being added to Melee.
I think Tingle is probably the most deserving, and we know Sakurai is okay with him, however EXPECTING HIM is something I'm not doing. I think he's a possibility. He's got moveset potential, he's prominent, and he's important. It's just a matter of Sakurai upgrading him from AT to playable character. Which, I have a hard time seeing happen, as much as I want it.
Then we have Ghirahim. Ghirahim COULD ride the SS train, but it's unlikely IMO. He is a one shot villain. But he does have an upperhand on Rosalina, considering his moveset practically writes itself.
Still, I think Tingle and Toon Zelda have a better shot.
And no, I would rather discuss a franchise for a few days if it means I have more votes. I'm not rushing this thing. It's more fun to have more people involved, even if it moves slowly.