Perhaps she is a time - traveler from the future. She has knowledge of future events that will endanger the ninja world and Sasuke. So, she tries to enlist his help, and in the process, a relationship develops between them. She might have unique abilities due to her time - traveling origin.
Well, it could be about the new girl being a powerful ninja from another village. She might initially be seen as a threat by Sasuke, but as they interact more, they discover they have common goals, like protecting the ninja world from a new emerging evil. And they gradually form a bond.
Female Sasuke might have enhanced stealth abilities. In some ninja cultures, female ninjas were known for their grace and ability to blend in, so she could use this to her advantage. Also, she might develop a more intuitive fighting style, relying on her instincts more. Since she's now a girl, her chakra control could potentially be more refined, as some fanfictions depict female characters having a different kind of connection to their chakra.
She could be more nurturing. Since girls are often associated with nurturing qualities in many cultures, Sasuke as a girl might show more compassion towards her friends and comrades. Maybe she would be more in tune with her emotions and not suppress them as much as she did when she was portrayed as a male. This could lead to more complex emotional interactions with other characters.
Sasuke could also have a complex mix of emotions. On one hand, he has a long - standing relationship with 'Naruto' as a male friend. Now that he knows Naruto is a girl, he might start to feel a different kind of connection. He may become more protective, but also a bit confused about his own feelings towards this new - found knowledge. He might even feel a bit betrayed that Naruto has hidden this fact for so long.
One great new Sasuke and Sakura fanfiction is 'A New Beginning'. It focuses on their post - war life and how they slowly rebuild their relationship. The author does a wonderful job of showing Sakura's growth and Sasuke's internal struggles as they come together.
Another theme is the way she is perceived in the ninja world. As a girl, she may face different expectations and prejudices. Some fanfictions explore how she overcomes these and still manages to be a strong ninja. In many of these stories, she has to prove herself more because of her gender.
There are quite a few fanfictions with such a concept. You can try searching on popular fanfiction websites like FanFiction.net or Archive of Our Own. Just type in relevant keywords like 'Sasuke as a girl' and you might find some interesting reads.
One interesting 'Sasuke is girl' fanfiction could be about her training differently as a female. Maybe she has to deal with different expectations in the ninja world. For example, she might face more discrimination but still rise above it to become a strong kunoichi.