There could be an older, more mature character, like a college student who she meets by chance and who is drawn to her unique charm and innocence.
Another option could be a fellow girl who starts off as a rival but then realizes she has feelings for Nagisa as they compete in something like a debate or an art competition.
In a fanfiction where Nagisa turns into a girl, it could be part of a wish - fulfillment scenario. Nagisa secretly wished to experience life as a girl for a day, and some supernatural being grants that wish. As a girl, Nagisa has to navigate through daily life, school, and social situations. There could be humorous moments when Nagisa tries to act like a girl but still has some of the old habits from being a boy, like being too rough during sports in a girl's body.
One potential love interest could be Joey. They could have a different kind of dynamic as Yugi is a girl. Maybe Joey is initially protective of her and then gradually falls in love. Their relationship could be based on mutual respect for each other's dueling skills and shared experiences in the Duel Monsters world.
The plot could center around Nagisa's relationships with the female students in the class. Since she has now become a girl, she might form a closer bond with them. They could start a girl - only club or group within the class where they share secrets and train together. However, this could also lead to some jealousy or conflict from the male students. Nagisa would then have to navigate through these relationship issues while still focusing on the main goal of assassinating Korosensei.
Perhaps a mortal with a special gift for metallurgy or some other craft related to Hephaestus' work. This mortal could be seen as an equal in terms of skill, at least in a way that catches Hephaestus' eye. They would bond over their shared love for the craft, and over time, this bond could turn into a deep, loving relationship. Hephaestus might be drawn to the mortal's unique perspective and the way they approach the craft despite not having the same divine powers as he does.
Sakura could be a potential love interest. Since Naruto has always had a connection with her, as a girl, that connection could turn into a yuri - like romantic relationship. They already know each other well, and Naruto's new form could make Sakura see her in a different light.
One possible fanfiction could be about Nagisa, as a girl, having to deal with the unique challenges in the Assassination Classroom. Maybe she uses her newfound femininity to her advantage in different assassination attempts. For example, she could play on the stereotypes that others might have of a girl being less of a threat, only to surprise her opponents.
Hinata is a very obvious choice. She has always had deep feelings for Naruto. In a fanfiction, their relationship could spiral further, exploring more of their shared interests, like their love for the village and their dreams for the future of the ninja world.
Well, a possible plot could be that Nagisa, as a girl, discovers new powers or abilities that are related to her femininity. Maybe she has a special intuition that helps the class in their assassination attempts. The story could also focus on her love life. She might develop feelings for a new character or perhaps Karma in a different way than in the original story. There could be a lot of drama as she tries to balance her feelings with her duties as an assassin in training.
Well, one possible plot could be that they face unexpected challenges. Maybe they're still in school and have to deal with the reactions of their friends and family. They might struggle with how to continue their studies while preparing for a baby.
If it's a crossover with the Percy Jackson universe, Percy himself could be interested in Ruby. They both have a sense of adventure and a desire to protect their friends. Annabeth might find herself drawn to Weiss because of their shared intelligence and strategic thinking. Leo Valdez could have a crush on Yang due to her mechanical skills and fiery personality. And Nico di Angelo might be intrigued by Blake because of her mysterious nature. These could all be great potential love interests in a RWBY - Percy Jackson crossover harem fanfiction.