According to Japanese Nintendo magazine Nindori, Rosalina was intended to appear in the recently released Mario Tennis Open. Despite all the different Galaxy elements and references like the Comet Observatory court, fans of the character were ultimately disappointed when she did not appear in the final game.
The translation is rough, but the article states that there was some sort of issue that did not allow Rosalina to appear in the final game. They also seem to be making some sort of comparison between Birdo from Mario Tennis on the Nintendo 64 and Rosalina.
If anyone is able to to give us a complete translation of the article, please contact us through the comments or email!
Thanks to NM4000T4 for the tip!