It could be that they are lured away by an outside force. In fanfiction, there could be a powerful mage or a rival guild that offers them something they can't refuse. This could be better training opportunities, more power, or a chance to solve a mystery that is important to them. Maybe they are promised to find out more about their pasts or something related to their magic that Fairy Tail can't provide.
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.
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.
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.
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.
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.
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.
The story could focus on their journey to find a new place to belong. They might meet new allies along the way who have different values and magic abilities, which would add new elements to the story. For instance, they could encounter a group of mages who use music as their form of magic.
He could kidnap her because he believes she's in immediate danger. Natsu is very impulsive and might not have time to explain things properly, so he just takes her away to keep her safe.
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.
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.
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.