I have a suggestion. Since this is a game modeled in a retro way, to me adding Rosalina in before other characters this early would feel a bit out of place. I feel like a Shy Guy or DK Jr. would be more true to the sprites from this era.
There are also a lot of new faces like a Blooper, Pokey or Thwomp i'd love to see before you put in a character we currently see appear in everything. I attached an image below with some ideas 