In many 'natsu 4th guild master fanfiction', the theme of change is prominent. The guild will surely change under Natsu's leadership. There's also the theme of responsibility. Natsu has to be responsible for the well - being of the entire guild now. And often, there's a theme of legacy, as he is following in the footsteps of previous guild masters and has to make his mark on the guild's history.
Well, there's a fanfiction called 'Natsu's Guild Mastery'. In this one, Natsu as the 4th guild master starts implementing some really unique training methods for the guild members. It shows his growth from a brash fighter to a responsible leader. It's really engaging as it also delves into his relationships with the other members in a new light.
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.
The dark guilds would be great antagonists. They might see Natsu's new leadership as a threat and try to undermine his guild. They could send their best mages to disrupt the guild's operations and steal important magical artifacts.
Maybe in the 'natsu guild master fanfic', Natsu becomes guild master during a time of crisis. A dark magic is spreading and threatening all the guilds. Natsu has to make difficult decisions, like forging alliances with former enemies. He uses his fiery determination to inspire his guild and they fight back the dark forces together. He also discovers new aspects of his magic along the way to better protect the guild.
Erza would be more composed. She would understand Natsu's reasons to a certain extent. However, she would also see the importance of the guild unity. So she would try to balance between respecting Natsu's decision and finding a way to make him come back if possible. She might send out some members to keep an eye on Natsu from a distance.
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, one possibility could be that Natsu leaves the guild because of a personal mission. Maybe he discovers a dark secret about his past that he believes can only be solved alone, so he leaves Fairy Tail to find out the truth without putting his friends in danger.