I do think there is a good chance of Toadette making an appearance. But I also think Rosalina would be an amazing playable character. And like I;ve said earlier, besides the whole height issue, I can't really come up with any reasons AGAINST her making an appearance.
I wouldn't mind if she was a playable character, but I kind of think her taking over the role Donkey Kong has assumed since Mario Party 5 would be better. I'd enjoy a "Rosalina Cosmic Challenge" or "Rosalina Galaxy Game" type of space where single-player and multiplayer games could be played with the prize being coins or a star, like the Donkey Kong games in Mario Party 7. Every mini-game would have a space theme to it, replacing the jungle theme that accompanies the Donkey Kong games.