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.
One possibility is that they leave to explore a new land filled with unknown magic. They might be on a quest to find a powerful magic item that could help Fairy Tail in the future.
One idea could be a story where Natsu and Juvia team up for a mission. They initially clash because of their different personalities, Natsu being hot - headed and Juvia being more reserved and love - stricken for Gray. But as the mission progresses, they start to understand and respect each other's strengths, and form a strong friendship. For example, Natsu could learn about Juvia's water magic and how she can use it in creative ways, and Juvia could be impressed by Natsu's determination and never - give - up attitude.
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 really interesting fanfiction is 'Juvia, Natsu and the Lost Magic'. Here, Juvia and Natsu are on a quest to find a lost magic. Along the way, they encounter many challenges. Juvia's water - based magic and Natsu's fire magic create some really cool and unique battle scenes. The story also has a sub - plot about their friendship growing stronger, and how they come to rely on each other in ways they never expected.
Well, they could have an interaction where Natsu accidentally sets off Juvia's temper. Maybe Natsu accidentally melts some of Juvia's precious water - made items. Juvia gets mad, but Natsu's cluelessness about how much he's upset her is kind of endearing. Eventually, Juvia forgives him when she sees how he was just being his usual, careless self.
Perhaps they leave because of a disagreement within the guild. Maybe there's a new policy or a decision made by the guild master that they don't agree with. For example, if the guild decides to get involved in a war that they think is unjust, they might choose to leave rather than participate against their beliefs.
One plot idea could be that Juvia and Natsu are forced to team up on a mission in a dangerous, enchanted forest. Along the way, Juvia's feelings for Gray start to shift towards Natsu as she sees his determination and kindness in a new light. Another could be a body - swap situation where Juvia and Natsu swap bodies and have to learn to deal with each other's powers and personalities, leading to some comical and heartwarming moments. A third idea is that in an alternate universe, Juvia and Natsu are childhood friends, and they have to face the challenges of growing up and protecting their world together, with their relationship evolving from friendship to something more romantic.
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 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.
One unique plotline is where Juvia and Natsu accidentally swap magic powers. This leads to a lot of chaos as Juvia tries to control Natsu's fire magic and Natsu struggles with Juvia's water magic. They have to learn to adapt quickly to this new situation while also dealing with the threats that come their way.