Well it doesnt really need to happen tbh.
i mean, Rosalina is a main character now , its pretty obvious. However she has considerably fallen into the ´´spin off´´ cast status during the recent years.
Peach getting her own game is pretty understandable as she is the most important Mario female , as well as one of the most recognizable and important characters overall not only in the series but for the company as a whole and also the videgame world.
I dont see why Rosalina should get a complete game for to be honest. She is always gonna be a stand out character , very special to me and to many but i dont see how that compares to any other actual Nintendo mascot character.
And no Rosalina is not a mascot character, Mario is , Peach is and Bowser is. Rosalina is not.
Saying she is would make tons of other popular characters in videogames actual mascots for companies and that's just very incorrect. The actual mascots represent their companies world wide since AGES ago , tied up with beng massively popular like Mario himself.
In the end, whether this ens up happening or not is up to Nintendo. But i dont see why.
The transition of Rosalina being NPC in Galaxy and then turning playable in MKWii didnt feel forced as introducing a New racer from a newer game at that time was just right.
then the transition to become more prominent was just expected as she grew really popular and i guess one more female would make the general standard future rosters look a bit more diverse.
But having her own game? After literally just being in party games mostly and having only 1 major main game appearence? (3d world doesnt really count as Rosalina's relevancy and character shine only in Galaxy 1)
No thank you. It just seems so wrong and unrealistic.
A mascot character has nothing to do with how long they've been around.
A mascot character is a character that's either originally designed with the intent to sell merchandise, or goes on to be used for that purpose. They're a marketing tool.
I have three different Rosalina plushies, two unique amiibo (three copies of the Smash one, two copies of the Super Mario one), a Rosalina poster, five non-amiibo Rosalina mini-figures, and hell, a Rosalina facecloth.
A facecloth.
That's not even getting into the more minor merch I don't have like trading cards, or the unofficial fan merch.
There is no beating around the bush with it, Rosalina is a mascot character. You put Rosalina in something or you make something in her image, and it prints money. Her Smash Bros amiibo is still one of the most highly sought after amiibo.
The thing that separates Rosalina from the other Mario characters is that she was designed as a character first, mascot second, by a creator who cared about her. All of the other Mario characters just take a super basic trait to define themselves and that's how they were designed from their inception. They all have a one-note, marketable gimmick at the core of their character that people can project themselves onto and that's how the sell merch (Mario is the jolly hero, Luigi is the underdog, Yoshi is adorable and child-like, Donkey Kong is dorky and goofy, Wario is greedy, Peach is the girl, etc). While other writers and developers have had their own takes on the characters that flesh them out more, that's only because the characters are so basic that they are basically blank slates.
Rosalina is different. She was personally constructed by Koizumi as a passion project of sorts, and wasn't constructed from just one trait, but has a variety of elements that define her as a character from the get go. She has agency. She feels like an actual character.
However, Nintendo realised that if you make an actually well-realised character like Rosalina, and they become beloved, it's an endless gold mine for money. So that's why Rosalina gets more merch and appearances now, even though that wasn't their initial intention. So yeah, long story short, Rosalina wasn't designed to be a mascot character from the concept stages, like every other main Mario character was. But she ended up being merchandised in that same way because her being a well-realised and fleshed out character sort of achieves the same result as building a basic, blank slate character from the get go, would achieve.
As for her own game, there's no reason not to do it. There's tons of potential premises. The Yoshi and Wario spin-off franchises derive their gameplay premises from the Mario games they originate from. Super Mario World was more exploration-based than other Mario games, so the Yoshi games are all about exploration. Super Mario Land 2 was a fairly action packed and fast paced game with a focus on collecting coins, so Wario games take those concepts to a more extreme degree.
The main gimmick of Super Mario Galaxy was the floatiness of being in space on little planetoids you could walk around. It was all about gravity. Rosalina already has gravity powers. So the gameplay could just revolve around controlling gravity to move around the environment.
As for the story, Rosalina transcends the existence of literal universes. Who knows what she's lived through and what she's seen? They can literally do anything with that premise. Plus they could always shed light on her family, or on how she got her powers, or on just how powerful she can get. If anything, Rosalina has much more solid premises to work off of than any other Mario character, past or present, when it comes to potential for her own game.
I mean the Dil...er, I mean, the Vibe Scepter, was completely made up for Super Princess Peach. They had to make up a power for Peach to serve as her core mechanic because she didn't have anything besides the float to make her unique (and the float isn't enough to make a game out of).
Rosalina already has a robust variety of powers to work with for game mechanics. You don't even need to make anything up for her.
is certainly one of them (though I also main 
