Luigi would probably be very surprised at first. He might be a bit confused, but then he would support his 'sister' Mario. Princess Peach would be delighted. She could finally have a female friend who has all of Mario's skills. Bowser might not even notice at first, thinking it's just another ploy by Mario. But when he realizes it's a real transformation, he might be a bit more hesitant in his attacks, not sure how to deal with this new female Mario.
The Toads would be quite shocked. They are used to seeing Mario as a male hero. But they would quickly adapt and start referring to Mario as a girl. Yoshi might not really understand the concept of gender transformation, but he would still recognize Mario's scent and be his loyal friend. Donkey Kong would probably have a good laugh at first, but then respect Mario's new form and still see him as a worthy opponent or friend depending on the situation. And the Koopa Troopas might be a bit more scared of this new female Mario, thinking she has some new, unknown powers.
Well, one possible plot could be that Mario touches a magical item in the Mushroom Kingdom. This item has the power to transform him. As he turns into a girl, he discovers new abilities. Maybe he can now access areas only females can in the kingdom. He also has to deal with how his friends like Luigi react to his new form.
Luigi could be an important character. He might notice the shy girl before Mario does and tries to encourage her to talk to Mario. Or, Toad could be there as a friendly sidekick, helping the shy girl get used to the new environment in the Mushroom Kingdom. He could also give her tips on how to overcome her shyness based on his own experiences.
One interesting plot could be Mario, now as a girl, having to navigate the Mushroom Kingdom in a different way. Maybe she discovers new powers that come with her new form and uses them to save the kingdom from a new threat.
Another possibility is that a group of Koopa minions could be the main antagonists. They might be under the orders of Bowser, but they have their own little schemes going on. They could target Mario - turned - girl because they think she'll be an easier target in her new form, but they are in for a big surprise when she shows her strength.
The fanfic has also led to more discussions within the Mario fandom. Fans debate about the plausibility of the relationships and storylines in the 'bowsette and mario' fanfics. This kind of interaction keeps the fandom alive and engaged, as everyone has their own opinion on how the Mario universe can be expanded or re - imagined.
Well, Mario could teach Naruto about the power - ups in his world. Naruto, being curious, would be eager to try them out. For instance, when Naruto eats a Super Mushroom, he might gain increased strength and speed in a way similar to his Sage Mode but with a Mario - style twist.
Nagisa's best friend might be really surprised at first. But then, being a true friend, would support Nagisa through this strange change. They would probably help Nagisa with things like choosing girl - appropriate clothes or dealing with any mean comments from other students.
Riku would probably be really confused at first. He might act a bit awkward around Sora, not sure how to treat this new female version of his friend. Kairi, on the other hand, could be more empathetic. She would likely help Sora adjust to being a girl, like teaching her about makeup or different hairstyles if that's relevant in the story's world.
The other gumballs in the gumball machine might be very confused. They've never seen such a thing happen before. Some might be scared and think it's some sort of curse. The humans who find the girl - gumball could be initially shocked. But some kind - hearted ones might take her in and try to help her adjust. If there are any pets in the story, they might sniff her curiously, not really understanding what she is but sensing that she's different.
Sasuke might be extremely suspicious. He has a long - standing relationship with Orochimaru, and this sudden change would make him wonder if it's a new trick. He would keep a close eye on her to see if she's still the same threat.