Well, a common theme is often the alteration of relationships. As a girl, Kakashi's interactions with her students, like Naruto and Sasuke, would be different. There could also be a theme of self - discovery. The character might have to come to terms with being a girl in a world that has certain expectations of her gender. And, of course, there could be themes related to the challenges she faces as a female ninja in a tough environment.
Yes, there might be such fanfictions in the Naruto fan community. However, I haven't read a specific one myself. Usually, in these fanfictions, the author might re - imagine Kakashi's character as a female. This could involve changing not only the physical gender but also how the character interacts with others in the Naruto universe, like with Naruto, Sasuke and other characters. It's a creative way for fans to explore different aspects of the story and characters.
One possible plot twist could be that Kakashi, as a girl, has to deal with different social expectations in the ninja world. For example, she might face more restrictions in combat training because of her new gender. Another could be that she uses her new form to her advantage in undercover missions, fooling enemies who are less likely to suspect a female. Also, there could be a romantic plot twist where a character who previously saw Kakashi as a rival now has different feelings for her as a girl.
Perhaps in a moment of extreme emotional vulnerability, Kakashi herself decides to reveal her true identity. For example, if a close friend is in danger and she realizes that she can't keep up the pretense any longer in order to save them, she might come out as a girl.
Another plot could involve Kakashi's relationships. Since she is secretly a girl, her interactions with other characters, like Naruto and Sasuke, might take on a different tone. For example, she might have a more nurturing side that she has been hiding. She could also have a love interest that is initially confused by her masculine facade but later discovers her true self.
Often, Gai is paired with Kakashi in Hurt Kakashi fanfiction. Gai, with his unwavering belief in friendship and his eternal rivalry with Kakashi, can be a great character to interact with a hurt Kakashi. He might try to pull Kakashi out of his slump through their friendly competition. Also, Rin is sometimes included. Since she has a significant past with Kakashi, her presence in the fanfiction can either be a source of comfort or more pain, depending on how the story is written.
In such fanfiction, the Kakashi OC often has unique physical features. For example, they might have distinct hair color or eye shape that sets them apart.
Kakashi might be initially surprised. But then, he would probably use it as a teaching moment for Naruto. He could say something like 'Naruto, stealing is not the way of a ninja.' And then give him a punishment like extra training.
Often, Naruto is paired with Kakashi in such fanfictions. Their bond from the original series can be explored in new ways in the context of Kakashi's reincarnation. For example, Naruto might be the one to discover Kakashi's reincarnated form and help him adjust.
His relationship with Naruto might change. Naruto could be more protective of Kakashi as a girl. He might see her in a different light, like an older sister figure. With Sasuke, there could be some initial awkwardness. Sasuke, who is used to Kakashi as a male, might not know how to interact with him as a girl at first. And with Sakura, they could form a closer bond as girls, sharing things that only females in the ninja world might understand.