I agree, Sunshine is incredibly overrated. It's actually quite a bad game, far beneath Nintendo's standard. There's not a whole lot of exploration to do in it, either, since the game doesn't let you pursue any objective but the one you select when you enter each stage, with a small handful of exceptions. Any part of the stage that isn't involved in the "episode" becomes empty space with no purpose (unless you're a masochist and go looking for blue coins). Terrible, terrible game design.
I'm not sure Rosalina deserves her own series more than Yoshi and Wario, though. I think they're more or less on the same level. You know who does deserve his own series (or at least one 3D platformer)? Bowser.
My main thing with Sunshine was literally all of the optional content was absolutely terrible (who thought it was a good idea to put Mario inside of a pachinko machine?), and the mandatory content ranged from good to bad, but never amazing.
The reason I say Rosalina deserves her own series more is comparing her situation now to when Wario and Yoshi got theirs:
Wario and Yoshi both got their own games and as a result, extended franchises, literally a year after their debut as Mario characters, and only because they had gameplay ideas that they thought Mario wouldn't fit for. Before this, Yoshi was a completely optional power-up, and Wario was the antagonist of one game you barely even saw him in. Nintendo didn't even have a solid idea of how fans around the world received the characters, since the internet did not exist. At the end of the day, the best you could say about them as characters was that Yoshi was cute and marketable because dinosaurs were all the rage in the 90's, and Wario was just le gross farty burly man who could be used for a more combat oriented platformer. And then Yoshi even got into the original Smash Bros as an individual representative of his own franchise.
Compare this to Rosalina, who even when she debuted, was a revolutionary character for the Mario franchise. She wasn't playable, but in terms of the story elements of Super Mario Galaxy, she was pretty much the main character of the game, as most of the story revolved around her and the Lumas. Mario stories usually are nothing but the same song and dance routine, and Mario characters are usually nothing but one-dimensional archetypes when they debut and remain that way in most games. Super Mario Galaxy and Rosalina marked something different in that regard, because instead of the main focus being on Mario saving Peach, it's on Rosalina, the Lumas, and Rosalina's work as a cosmic deity. Rosalina's storybook also popularised a practice in the industry known as "non-intrusive storytelling", something Yoshiaki Koizumi specifically set out to do in order to prove Shigeru Miyamoto wrong, that storytelling has it's place even in a game like Mario.
It's almost been a decade since Rosalina made her debut, and since the "Rosetta" (Rosalina's Japanese name) trademark was registered. Along with just how much of an important character she was in Galaxy, she has now become a recurring character in the main Mario cast, even being playable in a main series installment, is in Smash Bros (after a fair amount of time passed for her to solidify as a popular character, unlike Yoshi and his Smash debut) as the most hyped first party newcomer for the game she debuted in, is pretty much the second face of Mario Kart now, and her Amiibo is one of the most sought after of the lot. Not only this, but she debuted in an age where the internet has become a big part of her lives, so people have been able to express the demand for a Rosalina game and just their overall love for the character and Nintendo can see it. Heck, in the Iwata (may he rest in peace) Asks for 3D World, they outright say that she was included in the game because of her popularity.
So her popularity is easily more apparent and easy to recognise (due to the internet) than Yoshi's and Wario's were when they got their games, without a doubt.
But my justification isn't just her popularity - quite simply, Rosalina has more purposes behind making a game or franchise about her. Wario and Yoshi were (and in Yoshi's case, still is) one dimensional characters when they debuted. Rosalina wasn't, she had a lot of agency, and one of the main reasons she is so popular is because of that, and because of the lore surrounding her. A franchise starring her would naturally be story driven and have a lot of lore, expanding on her character (canonically, she is immortal and centuries old, pretty much god of the Mario-verse...but we know she wasn't always like that, so a game about her could explain this, for example). Wario and Yoshi didn't have backstories that needed expanding on, they were randomly given motivations out of the blue for their games (Wario searching for riches, Yoshi helping the Baby Mario Bros out). Rosalina already has plenty of material to work with, the foundations are there.
As far as powers go, Wario and Yoshi had very simple powers in their debut games, both in the Mario franchise and their own franchises - Wario had brute strength, Yoshi had his digestive system. Rosalina is literally a goddess and Nintendo have pretty much shown they have no problem giving her random new powers in each game (for example, Super Mario Bros: Puzzles & Dragons Edition gave her the Water element and water powers, even though she had never been seen using water based powers before), so there is no limit to what they can do with her. If they wanted to narrow it down, her absolute control over gravity is her most iconic power, so I would suggest making an action-adventure game similar to Gravity Rush on the PS Vita, where she develops these powers from a younger age. Rosalina also has mechanical prowess and is enthusiastic about engineering, so that can factor in, maybe with Star Fox-esque gameplay segments when travelling between worlds, with a customisation element added in, or even just giving her small gadgets to use.
Also Yoshi and Wario are a cute animal mascot character and a tough male character respectively, something Nintendo had plenty of even at the time, so they didn't represent anything that was lacking in the company's franchises. However, giving Rosalina her own franchise where she is the main playable character would add another franchise where the consistent playable protagonist is a female character, something Nintendo SEVERELY lacks (Metroid is the only franchise they have that has one and is still active - and the next game in the series won't even star Samus now, it's going to have seemingly male Galactic Federation troops as the protagonists). And that would undoubtedly be received well since Rosalina is revered in part for how positive a role model she is for women.
Bowser deserves a game too, but I wouldn't say he deserves a franchise. Or rather, he doesn't need one. He's Mario's arch nemesis, so the Mario franchise is half his anyway.
