Rosalina will make her first appearance in a Mario Party game this winter in Mario Party 3DS. Appearing in the Super Mario Galaxy themed board, Rosalina can be seen standing near the end, right next to her Comet Observatory.
While this confirms that Rosalina will not be a playable characters from the start, there is still a chance that she can be unlocked at a later time. Regardless, Rosalina fans rejoice!