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.
It could lead to a change in the guild's dynamics. Naruto has his own unique combat style and abilities. His departure might make the guild realize the need to diversify their fighting strategies. For example, they might start to look for members with skills similar to Naruto's ninja techniques.
She would fit in very well. The Fairy Tail guild is all about family and acceptance. Given her parents are Lucy and Natsu, she'd be welcomed with open arms. She could learn from the older members like Makarov and Mirajane, and bond with the younger ones like Romeo. Her magic skills would also contribute to the guild's strength during battles or when helping the town.
The guild would likely be ecstatic. Gray would probably make some snarky comment about Natsu finally growing up, but deep down he'd be happy for his friend. Erza would be all for it and might start planning the wedding immediately, giving orders to everyone in the guild to help out.
The story would have to shift its focus. Instead of relying on Natsu's dragon - slayer magic and Mirajane's take - over magic, new characters or existing ones would need to shine. This could open up new plotlines such as the discovery of new magic among the remaining members or the forging of new alliances outside of the guild to compensate for the loss of Natsu and Mirajane. It would be a major turning point in the fanfiction's narrative.
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.
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.
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.
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.
The guild would be in chaos. The members would be shocked and divided. Some might side with Natsu if they believe there was a just cause, while others would see him as a traitor. There would be a power vacuum, and different members might vie for the position of guild master. This could lead to internal strife and potential break - up of the guild.
Well, in fanfiction where Natsu leaves the guild, he could become a sort of lone wolf. He might travel to far - off lands to discover more about his dragon - slayer powers or his past. This could lead to him meeting new and powerful allies or enemies. Without the support of the guild, he would have to rely solely on his own strength and wits. It could also make him a more serious and hardened character, as he no longer has the safety net of his Fairy Tail family.