If Lucy and Juvia leave Fairy Tail, it could change the dynamic of the relationships within the guild. For example, Gray, who has a complex relationship with Juvia, would surely be affected. Natsu, who is close to Lucy, might go on a quest to find her. Also, the guild's reputation could take a hit. Other guilds might see Fairy Tail as a place where members don't stay, which could affect future alliances and interactions.
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.
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 possible plot could be that Lucy and Juvia leave Fairy Tail to go on a self - discovery journey. They might want to explore their individual magic powers more deeply without the constraints of the guild. Maybe they travel to different magical realms, meet new characters, and learn unique magic techniques that they can bring back to Fairy Tail later.
A powerful magical being from another dimension could be the main antagonist. This being senses the departure of Lucy and Juvia from Fairy Tail and sees it as an opportunity to invade their world. It tries to stop the two girls from getting help or finding a way to stop it, as they are two of the most powerful mages outside of the guild protection now.
In some fanfictions, Juvia and Lucy could be in a situation where they have to rescue a kidnapped guild member. Juvia uses her water magic to track the kidnapper's location through the moisture in the air. Lucy uses her keys to call upon spirits that can help in combat and stealth. Along the way, they face many challenges and have heart - to - heart conversations. This experience deepens their friendship and they become closer allies within the Fairy Tail guild.
One possible reason could be to seek new adventures. They might feel they've reached a point in Fairy Tail where they've done all they can and want to explore new magic and challenges elsewhere. For example, they could have heard of a far - off land with a different type of magic that they are eager to learn about.
Midnight might use his illusions to make Lucy think she's lost her Celestial Keys. This would be a huge psychological blow to her. But then Lucy could remember the bond she has with her spirits and break through the illusions. It would be a battle of wills as much as magic.
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 fanfiction could also explore a scenario where Lucy and Juvia enter a magic tournament together as a team. They face various opponents and use their combined skills to win. Their opponents might underestimate them at first because they seem like an unlikely pair. But they prove everyone wrong with their strategic use of magic. Lucy could summon spirits to distract opponents while Juvia attacks with powerful water spells.
One possible reason could be that she feels she has outgrown the guild. Maybe she wants to explore new magic on her own and believes the Fairy Tail environment has become too comfortable and is hindering her growth.
Maybe they leave because they want to search for answers about their pasts. Happy might want to know more about his origins, and Lucy could be seeking something related to her family's history.