Often, it could be because of a false accusation. Maybe someone framed Natsu for a crime he didn't commit, like stealing a powerful magical artifact from the guild. So, to maintain peace and order, the guild leaders had no choice but to banish him.
In some fanfictions, it's due to his uncontrollable power. Natsu's dragon slayer magic can be extremely powerful and sometimes dangerous. If he loses control during a crucial moment and causes a great deal of damage, the guild might banish him to protect both the guild members and the general public.
Another reason could be a prophecy. There might be a prophecy that foretells Natsu bringing great destruction if he stays in the guild. Although it's just a prophecy, the guild takes it seriously enough to banish him in order to try and prevent the predicted disaster.
One popular 'Fairy Tail Natsu Banished' fanfiction might be 'Away from Home'. It explores Natsu's journey after being banished and how he overcomes various challenges on his own. He meets new characters and discovers new powers along the way.
One possible reason could be that he was wrongly accused of a serious crime within the guild. Maybe some powerful and influential members believed he was a threat and pushed for his banishment. For example, if there was a string of mysterious attacks and Natsu was framed as the culprit due to his often wild and uncontrollable nature in battle.
Well, a really interesting one is the fanfiction that focuses on the emotional turmoil of Mira and Natsu when they are banished. It shows how they struggle with being away from their friends at the guild and how they gradually find a new purpose together, but I can't recall the exact name right now.
He might become more brooding and distrustful. Banishment is a harsh experience, and Natsu, who was always so close to his Fairy Tail family, would feel betrayed. This could lead to him suppressing his emotions and becoming more solitary as he tries to figure out how to clear his name.
They have great chemistry. Natsu is the impulsive and powerful fire wizard, while Erza is the strong and disciplined armor wizard. Their contrasting personalities make for an interesting dynamic. They also have a long history in the Fairy Tail guild, which gives fanfiction writers a lot of material to work with.
You can start by looking on fanfiction - dedicated websites like Archive of Our Own or FanFiction.net. Just search for 'Mira x Natsu banished' in their search bars and you'll likely find a variety of relevant stories.
People write such fanfictions because they want to explore different scenarios for the character. Natsu is a popular character, and seeing him in a situation where he's left behind allows for a lot of character development. It gives the writer a chance to show his determination and resourcefulness outside of the normal guild setting.
One possible reason could be a past trauma. Maybe he witnessed something really horrible during a battle or an incident at school that shocked him so much he lost his ability to speak. For example, if he saw a friend in great danger and was powerless to help immediately, it could have had such a psychological impact on him.
Well, in some fanfictions, the author might want to explore a different side of Natsu. Maybe they think it adds drama. Natsu could be written as a jerk because of some past trauma that's not shown in the original series, which makes him lash out at everyone in the fan - created story.
Mirajane is a popular choice too. She has a mysterious side to her, and in the fanfictions, she could use her unique abilities to help Natsu or be involved in the events that the Fairy Tail members are watching. Also, Laxus might be included. His powerful magic and complex character can add depth to the fanfiction stories, especially when his relationship with Natsu and the rest of the guild is explored.