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 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.
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.
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.
First, you need to establish the reason for the transformation. It could be a magical spell gone wrong, or some kind of strange dragon - related phenomenon. Then, focus on how Hiccup adjusts to her new body. She'll have to deal with different physical abilities and limitations. For example, she might be physically not as strong as before, but more agile in some ways.
A fanfic could explore how Hiccup's transformation into a girl affects his friendships. Astrid might react in a complex way. At first, she could be jealous or confused, but then they could form a stronger bond. Also, Hiccup's father, Stoick, might have a hard time adjusting to his son becoming a daughter, which could add a lot of emotional depth to the story.
One possible scenario in this fanfic could be that Tony Stark, through some sort of magic or scientific experiment gone wrong, turns into a girl. This could lead to a whole new set of adventures and character developments. For example, his relationships with other characters like Steve Rogers (Stony) would be completely different. He might have to face new challenges in a different physical form, such as how society and his friends would accept him. Also, his own self - perception would change, and he'd have to re - evaluate his identity.
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.
A plot could involve the shy girl being a huge admirer of Mario from afar. She is too shy to talk to him directly but leaves little anonymous gifts for him. Mario eventually catches her in the act. At first, she's embarrassed but Mario is kind and encourages her to be more open. They end up going on an adventure together where the shy girl discovers her own hidden strengths.