I would like a game with Rosalina because her magic/gravity(?) powers and Luma are more defined and different than the rest of the Mario cast and at least have to potential to branch out into different gameplay mechanics like Yoshi and DK have. This is assuming they do it justice though instead of making it like...Super Princess Peach or something.
It's also kinda weird seeing the hate for her in this thread. Talking about Rosalina being Nintendo's Lighting when she's only been in a few Mario games/spinoffs of all things is the height of irony.
In addition, she's got more backstory than most so she's not a blank slaet, isn't particularly stereotypical, has a neat sidekick like Luma, has room to grow further as a character, and the game could be fun/different.