One possible plot could be Stiles' struggle to adapt to his new female body. He might have to deal with different social expectations and how his friends react to his new appearance. Another plot could involve Stiles trying to find a way to turn back into his male self while facing various challenges along the way.
There might be a jealous friend or acquaintance who feels that Stiles, as a girl, is getting too much attention. This person could then start spreading rumors or causing trouble for Stiles in an attempt to bring her down.
Scott would probably be very protective. He'd feel responsible for taking care of Stiles and making sure he doesn't get into too much trouble. Lydia might be initially shocked but then use her intelligence to try and figure out how to reverse the situation. And Derek, well, he'd probably be a bit annoyed at first but would also end up being one of Stiles' protectors.
In some fanfictions, a possible plot could be that Stiles, as a kid, has a different perspective on the supernatural world around him. He might not be as afraid as he was when he was older. For example, he could befriend a supernatural creature that the older Stiles would have been more cautious about. This new friendship could lead to new alliances for the pack. Also, his family's reaction to his sudden transformation could be explored. His dad might be really confused and worried, while his friends are trying to keep it a secret from the wrong people in town.
Well, often in these fanfictions, Stiles becomes more confident. His physical appearance might also change, like he could start dressing better or have a new haircut that gives him a 'hot' look. For example, he might start wearing more stylish clothes that show off his personality.
Often, the themes include adaptation. Stiles has to adapt to his new fox body, which might have different physical abilities and limitations. There's also the theme of mystery, like trying to find out how he got turned into a fox and how to reverse it. And of course, there's the theme of relationships. His relationships with his friends, family, and love interests will all be affected by this transformation.
Well, it could start with some magical or scientific accident that causes Robin to be transformed. Maybe he was messing around with a new gadget in the Batcave that had unforeseen side effects. Then, the story could explore how Robin has to adapt to life as a girl. He might face challenges like getting used to different clothing styles, or dealing with how people around him react. There could also be some comical moments, like when the other superheroes are really confused at first but then try to be supportive.
It might be due to some scientific experiment gone haywire. In a fanfiction world where science and ninja techniques are combined, an experiment that was supposed to enhance Sasuke's abilities could have had the unexpected side - effect of changing his gender. This could lead to all sorts of interesting plot developments, like how he copes with his new physical form and how his relationships with other characters change.
Often, it could be fans who are interested in exploring different aspects of the Link character. These might be female fans who want to see a different perspective on the typically male hero. They might be creative writers who like to play with gender - swapping concepts in fanfiction.
One possible plot could be that Ash wakes up one day as a girl and has to deal with all the changes in his daily Pokémon adventures. For example, he might have to adapt to new clothing styles and different social interactions. His Pokémon might also react in various ways to his new form.