It could be just for the fun of it. Writers often like to play with the established characters and create unique scenarios. In this case, seeing strong Naruto in a girl - like form can lead to humorous situations, like when his friends first see him and their shocked reactions. Also, it can open up new plotlines, such as Naruto having to disguise himself as a girl for a mission and having to navigate the situation while still being his strong self.
Often, when Naruto is made to look like a girl in fanfiction, his clothing might change to something more feminine. He could be dressed in a simple kimono - perhaps a light - colored one with floral patterns. This new appearance might cause different reactions from the other characters. For example, Sasuke might be initially shocked but then gradually see that it's still the same strong - willed Naruto underneath. And Naruto's strength could be shown in a different light. Instead of brute physical force, it could be more about his mental tenacity and strategic thinking, which are not affected by his new appearance.
One possible reason could be in the realm of artistic interpretation in the fanfiction. The author might want to explore a different aesthetic or a unique plot where Naruto has some physical or behavioral traits similar to Orochimaru for shock value or to create a new kind of character dynamic.
There could be a story arc where Naruto uses his new appearance as a girl to his advantage in espionage or undercover missions. This would bring in elements of mystery and intrigue. His interactions with other villages' ninja would also be different, and he could use this to gain information or form new alliances in a way that he couldn't as a male Naruto.
One possible reason is their physical appearance. They might have some similar facial features or hair color that could give the impression. Another reason could be their skills. If in fanfiction, Tobirama's skills are exaggerated and Naruto has similar - looking skills, it could lead to this idea. Also, their status in the ninja world as important figures might contribute.
One reason could be for the sake of creating unique storylines. By making Naruto look like a girl, it can introduce new romantic interests or change the dynamics of his relationships. For example, it might lead to different interactions with Sasuke or other characters.
Maybe Naruto was trying to protect Sasuke from a certain danger. There could be a curse or a powerful enemy that targets male shinobi. So, Naruto uses a risky jutsu to transform Sasuke into a girl to keep him safe. Or it could be that Naruto found a strange ancient scroll with a transformation technique that he thought was something else and when he used it on Sasuke, it turned him into a girl.
One reason could be for strategic purposes. Itachi was a highly strategic ninja. So, in fanfiction, Naruto might act like him to outwit a particularly difficult enemy. For instance, when facing an enemy who is a master of deception, Naruto could adopt Itachi's calm and calculating demeanor to see through the enemy's tricks.
It could be that Naruto is trying to prove himself. He wants to show that he can be as great a fighter as his father. Also, he might have received some training or knowledge transfer from someone who knew Minato well. This could have inspired him to adopt Minato's fighting style. He may also be in a situation where his normal fighting style isn't working, so he turns to Minato's as a last resort.
Maybe Naruto is put in a situation where he feels that Hiruzen's way of handling things is the best option. For instance, if there is a major crisis in the village that requires a more traditional and experienced approach, Naruto might choose to act like Hiruzen. Also, he could be influenced by the stories he has heard about Hiruzen and wants to fill his shoes in a way.
Maybe Naruto admires Kakashi's skills. Kakashi is a highly respected ninja, and Naruto might want to be as good as him. So he starts acting like him to learn from him.