In some fanfictions, Juvia could leave Fairy Tail because she gets an offer from another powerful guild. This offer might come with the promise of greater power, more resources for her to develop her water magic, or a chance to work with some renowned magic users. It would be a difficult decision for her, but she might see it as an opportunity she can't pass up.
Another reason could be that she believes she is a liability to the guild. Maybe after a particularly tough battle where she made a mistake that cost the guild, she decides to leave to protect them from her own potential mistakes in the future. She would rather sacrifice her place in the guild than put her friends in danger again.
She might leave due to heartbreak. For example, if she felt that Gray didn't return her feelings in the way she hoped, she could decide to leave to get over him and start anew.
A possible reason could be personal conflicts within the guild. Lucy might have an issue with some of the guild's rules or how certain things are run. Juvia could be following Lucy for some reason, like loyalty or because she also has her own issues with the guild. For instance, if there are tensions between Juvia and other members over Gray.
One possibility is that the guild would experience a significant emotional impact. Lucy is a central figure, and her celestial spirit magic has been crucial in many battles. Juvia, with her water magic, is also a strong member. Their departure might lead to a decrease in the guild's overall power. Some members might feel betrayed, while others could be motivated to find out why they left and try to bring them back.
One interesting Juvia Leaves Fairy Tail fanfiction could be one where she goes on a solo adventure to discover her true powers outside of the guild. In this type of fanfic, she might encounter new magical creatures and have to rely solely on her own skills.
A possible fanfiction could be that Juvia leaves to protect the guild. She discovers a dangerous plot against Fairy Tail and decides that her absence would be the best way to keep everyone safe. As she is away, she secretly works to thwart the evil plan, and in the end, she returns victorious, having saved the guild she loves so much.
He could leave because of a misunderstanding. Maybe he overhears something wrong and thinks the guild doesn't trust him or value him anymore. So in a fit of anger and pride, he decides to leave.
One possibility could be that they leave to seek greater magical power. Maybe they've heard of a mysterious land filled with powerful magic and they decide to embark on a journey together. They leave Fairy Tail behind in order to become stronger and return to protect their guild in a more powerful way.
Juvia's character often develops through her relationships. For example, if she has a romantic relationship in the fanfiction, she might become more confident and assertive in expressing her feelings. She could also develop by facing her fears. If she's put in a dangerous situation, she might overcome her insecurities and grow as a mage.
One fanfiction I remember has Juvia leaving after a big fight in the guild. She feels she doesn't belong anymore. As she leaves, she meets a group of outcasts who teach her new ways of using her magic. Eventually, she returns to the guild with a new perspective and stronger magic. It's a really engaging story that shows Juvia's journey in a unique way.
Another theme is regret. Gray usually experiences regret for not seeing Juvia's love earlier. This can lead to his attempts to make things right, which adds a lot of drama to the story. For example, in many fanfictions, he tries to win her back in various ways.
One good fanfiction could be 'The Missed Chances' where it delves deep into Juvia's unrequited feelings and how Gray's realization comes a bit too late. It shows their complex relationship through different scenarios and dialogues.