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.
One possibility could be that Natsu is under some sort of dark magic influence. Maybe a powerful enemy has managed to control his mind, making him turn against the guild master. Another reason could be a misunderstanding. If some false information is fed to Natsu about the guild master being a threat to the guild or his friends, his hot - headed nature might lead him to take drastic action without fully understanding the situation.
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 way could be through some kind of time - travel or alternate reality plot. For example, Natsu could find a way to go back in time and prevent his actions. This would allow the story to get back to the more positive and friendly tone of the original Fairy Tail.
Well, in some fanfiction, it might be used as a shock value plot device. However, it goes against the general nature of the relationship between Natsu and the guild master in the original Fairy Tail series. The series is mostly about friendship and loyalty within the guild, so such a plot would be quite a deviation.
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.
The guild would likely be shocked at first. They consider Natsu as one of their own, so his attack would be completely unexpected. Some members might try to talk him down, like Lucy. She has a close bond with Natsu and would probably try to reason with him.
Another great choice could be Laxus. He has that tough - guy exterior but also a lot of room for character growth. In a fanfiction as guild master, he could bring a different leadership style, more aggressive perhaps. He might change some of the guild's traditions while still maintaining the core values of Fairy Tail. He also has a great relationship with his team, which could be further developed in the fanfiction.
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.
In some fanfictions, Natsu as the 1st master of Fairy Tail might be depicted as a powerful and visionary leader. He could be shown as someone who laid the foundation for the guild's values of friendship and strength.