People actually guessed a playable Rosalina after getting a look at a screenshot that showed outlines of all the possible stamps for use on Miiverse. That screenshot showed that there were five icons that resembled the player character icons used in the user interface (Player selection and life counter).
The Other Darrin: Rosalina is voiced by Laura Faye Smith rather than her previous voice actresses Mercedes Rose and Kerri Kane. According to MarioWiki, Kane and her family moved.
Throw It In: The Double Cherry powerup was created by accident when a developer placed two Marios instead of one. The team loved the concept so much they decided to add it in.
What Could Have Been: Peach was not always intended to be playable. At some point, she would still be the Damsel in Distress, and the Sprixie Princesses wouldn't have existed if that decision was made. But then Yoshiaki Koizumi suggested to have her within the playable roster.