A common theme is identity crisis. Sasuke suddenly being a girl would make him/her question who they are now. Another theme could be acceptance, both from Sasuke herself and from the other characters. Also, there might be a theme of newfound friendship. As a girl, Sasuke might make new friends that he wouldn't have made before.
In addition to the obvious ones like Naruto and Sakura, other characters from Sasuke's clan might interact with him. For instance, if there are any remaining Uchiha members, they might have mixed reactions. Some could be shocked and try to figure out what happened, while others might be more accepting. Also, characters like Shikamaru, who is very observant, would surely notice the change in Sasuke and might have some unique interactions, perhaps trying to figure out how this change could affect their team dynamics or the overall situation in the village.
She could also change in how she approaches relationships. Before, as a boy, Sasuke was more reserved in his interactions with others, especially with Naruto. But as a girl, she might be more forward in expressing her feelings towards Naruto, whether it's friendship or something more. This could lead to some really interesting character development arcs and new plot directions in the fanfiction.
Well, perhaps a plotline could involve Sasuke turning into a girl and using this new form to his advantage in some undercover mission. He could infiltrate an enemy group more easily as a girl. Also, there could be a love triangle where some of the characters who previously saw Sasuke as a male now have different feelings towards him as a girl, which would create a lot of drama and conflict.
For Naruto, he might become more protective of Sasuke as a girl. Their friendship could take on a different dynamic, with Naruto being more chivalrous perhaps. Sakura might initially be jealous but then they could form a stronger bond as girls sharing common experiences. Kakashi would probably have to adjust his mentoring style to account for Sasuke's new situation.
Well, a possible plot could be Sasuke's transformation into a girl is part of a powerful jutsu gone wrong. As a girl, she has to find a way to reverse it. In this process, she might discover new aspects of her own abilities or weaknesses. Also, there could be a plot where she has to prove herself in battles all over again, since some might underestimate her as a girl.
Perhaps the story could explore how the other characters react. The girls in the village might be more accepting of 'female Sasuke' and try to befriend her. The boys, on the other hand, might be a bit jealous of Naruto's close relationship with her. And this new form of Sasuke could also bring out new abilities or powers, or at least a different way of using her existing skills, which would make for some interesting battles and plot twists.
Well, often in such fanfiction, the change might lead to new relationship dynamics. Sasuke as a girl could have different interactions with Naruto, for example. Maybe there would be more emotional and less combative exchanges at first. The story could explore how Sasuke deals with this new form and the reactions of other characters in the Naruto universe like Sakura, who might be jealous or more protective.
Well, with Kakashi, he might be more cautious in how he trains Sasuke. As a girl, Kakashi might have to consider different aspects of training, like physical limitations that might come with the female form. His relationship with his clan members, if they were still around, could also be different. They might have different expectations of a female Sasuke. And when it comes to the other male ninja in the village, some might be attracted to him, which would create some awkward or interesting situations.
Well, usually it starts with Naruto using some sort of jutsu or magic (in the fanfic world). Then Sasuke suddenly finds himself in a female body. At first, he might be really confused and angry. But as the story goes on, he starts to experience things differently as a girl. For example, he might notice how people treat him differently. Naruto might also start to see Sasuke in a new light, and there could be some really interesting character development and new relationships forming between them.