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.
Perhaps they leave because of a misunderstanding within Fairy Tail. They could think that they are not truly accepted or that their way of doing things is not in line with the guild anymore. So, they set off to prove themselves elsewhere. They might face various challenges on their own, like dealing with powerful dark guilds or solving mysteries that no one else in Fairy Tail has been able to. In the process, they grow stronger both individually and as a duo.
A powerful dark wizard who has a vendetta against Natsu. Maybe he blames Natsu for something that happened in the past, like the destruction of his dark guild by Fairy Tail. So, when Natsu and Happy leave Fairy Tail, he sees it as an opportunity to get revenge.
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.
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 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.
It could be about Natsu having left Fairy Tail for some reason and then reuniting with the guild in a very unexpected way. Maybe he has new powers or a different outlook on life that causes some friction and also some amazing moments of reunion and growth within the guild.
Well, one possibility is that Natsu could join Fairy Tail in a different way than in the original story. Maybe he has some special mission before officially becoming a member. For example, he has to prove his loyalty by helping an elder member with a personal problem.
One possibility could be that Natsu is attacked due to a misunderstanding within the Fairy Tail guild. Maybe a new member misinterprets Natsu's actions as a threat and attacks him in self - defense.
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 possible plot could be that there is some sort of misunderstanding or dark magic at play that causes Natsu and Mavis to fight. Natsu, being hot - headed, jumps into the fray without fully understanding the situation. Mavis, on the other hand, is trying to figure out a way to stop the fight without harming Natsu. As the battle progresses, they both start to realize that they are being manipulated and then team up to face the real enemy.
One possible plot could be that the guild goes into deep mourning. Gray might blame himself, thinking he could have protected Natsu. Lucy could be in a state of shock and disbelief, and she might start writing in her diary about all their adventures, as if reliving the memories to deal with the pain.