One possible plot could be that Naruto has to pretend to be a girl for a mission. Maybe he needs to infiltrate an all - female group. He would struggle with things like dressing up as a girl, learning feminine mannerisms, and trying not to blow his cover. For example, he might accidentally use his male - voice or do some typically male actions like roughhousing. This would lead to some comical situations as he tries to keep his identity hidden from the other girls in the group.
Sakura might be really surprised at first. She could offer to help Naruto with his girl disguise, teaching him things like how to do his hair or choose the right clothes. She might also find it a bit funny but would be supportive in the end, seeing it as part of a mission. Sasuke, on the other hand, might be a bit more aloof. He would probably just raise an eyebrow at Naruto's new look but wouldn't make a big fuss about it as long as it was for a good reason.
The most common theme is identity crisis. As the girl pretends to be a boy, she often questions who she really is. Is she still the same girl inside or is she becoming something else? Another theme is the power of disguise. It shows how a simple change in appearance can lead to different opportunities and experiences. And finally, the theme of equality is there. By being able to function as a boy, it shows that girls can be just as capable, which is an important message in these types of fanfictions.
There's also 'Twelfth Night' by William Shakespeare. Viola pretends to be a boy named Cesario to work in a strange place. The plot gets quite interesting with all the complications that arise from her disguise.
It might be an adventure story. The girl could use her disguise to explore new places or get involved in exciting escapades that she wouldn't have been able to otherwise.
One idea could be a crossover where Naruto enters the Incredibles' world and teams up with Elastic Girl to fight a new villain. They could combine Naruto's ninja skills and Elastic Girl's stretching abilities to defeat the bad guy.
One popular 'girl pretends to be a boy' fanfiction is 'The Girl Who Became a Boy for a Day'. In this story, the girl has to pretend to be a boy to enter a male - only competition. It's full of humorous situations as she tries to fit in with the boys, like learning how to walk and talk like them. Another one could be 'Masquerading as a Boy', where the girl disguises herself as a boy to protect her family's honor. The story unfolds with her facing various challenges and making new friends along the way.
Maybe it could be a new character created specifically for the fanfic. This new girl could be from a traveling group of merchants who Naruto meets during one of his missions. Their encounter could lead to a relationship and ultimately the pregnancy.
One reason could be to protect his friends. Naruto doesn't want his friends to worry about him, so he pretends to be happy even when he's facing difficulties. For example, when dealing with the threats from powerful enemies, he shows a happy face to keep his team's morale up.