Perhaps a powerful demon that has a grudge against Galuna Island. The demon sees the girl as a key to getting revenge on the island. Natsu, being the hero, would stand in the way. The demon could have minions that constantly try to capture the girl, creating a lot of action - packed scenes in the fanfiction.
One idea could be that they go on a quest together to find a rare magical item that is said to have the power to save Galuna Island from a new threat. Another could be that the girl from Galuna has a special power that Natsu needs to unlock his own new level of magic. Maybe they start off as rivals, competing to be the best in a magical tournament but then gradually fall in love as they discover more about each other.
Natsu is often seen as a strong and passionate character. His determination in battles and his loyalty to his friends make him an interesting subject for fanfiction. Writers can explore different scenarios where he shows his strength or his softer side.
Another possibility is that Natsu is turned into a girl by a magic spell gone wrong. As a girl, he discovers new magical abilities or a different way of using his existing fire magic. Maybe the transformation also affects his personality slightly, making him more empathetic and in tune with the feelings of others around him.
Gray would be extremely angry. He would immediately start looking for clues as to where Natsu was taken. His ice - make magic would be on full display as he might freeze things in frustration while trying to figure out the situation.
One possible plot could be that the kidnappers are from a rival guild who want to use Natsu as leverage against Fairy Tail. Gray could then embark on a solo mission to rescue Natsu, facing various traps and enemies along the way. He might have to use his ice - make magic in creative ways to overcome obstacles.
If Natsu is a girl in this fanfiction, her character might change in terms of her relationships. For instance, she may be more self - conscious about her appearance compared to when she was male. Also, her interactions with male characters like Gray could shift. Instead of their typical rival - like banter, there could be a more complex emotional undertone, like a friendly - rivalry with a hint of romantic tension perhaps.
Natsu's friends like Gray, Erza, and Lucy are often involved. Gray might be constantly teasing Natsu about the transformation, creating some funny banter between them. Erza could be the one who tries to keep order and protect Natsu in this new form. Lucy might offer more emotional support, sharing girl - related experiences with Natsu to help her adjust.
He could also face challenges within his own guild. Some of the members might tease him, while others might be overly protective. Natsu as a girl would have to navigate through these new social dynamics. Also, his appearance as a girl might attract unwanted attention from some villains who see him as an easier target, but he would prove them wrong with his strength.