I would consider myself a Rosalina supporter, but I'm also not holding my breath. She is by far my favorite Mario character and I am super excited to play 3D World because she's playable in it, but I feel that I would be setting myself up for disappointment if I expect her. However, if she keeps on being a mainstay Mario character like she has since Galaxy, maybe my sentiments will change.
But, that's just my pessimistic view on her inclusion. She would be totally awesome with a Galaxy-based moveset around gravity and cosmic magic. If she actually makes it, I can definitely see myself maining her.