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.
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.
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.
He might lose his friends at Fairy Tail. Since Fairy Tail is all about friendship and light, joining a dark guild would put him at odds with his former comrades.
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.
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, one possible plot could be that Natsu, after some sort of betrayal in Fairy Tail, decides to create his own dark guild. He might gather some disgruntled mages who also feel wronged by the mainstream guilds. Their goal could be to change the magic world's power structure in a more radical way, using darker forms of magic that are usually shunned. For example, they could explore ancient and forbidden magic spells to gain an edge over other guilds.
Gray would be shocked at first. He and Natsu have a long - standing rivalry - but also a deep friendship. He might initially be angry that Natsu would just leave like that, but then he would realize how much he'll miss him. He might also feel a sense of loss for the guild as a whole, since Natsu is such an important part of it.
Well, the two main characters are Natsu and Lucy. Natsu's departure is the inciting incident. Lucy, now pregnant, has to face the world on her own. Her character will probably show a great deal of growth as she deals with single motherhood. While the story is mainly about them, other characters from their guild might also play important roles, such as providing support or causing additional conflict in relation to Lucy's situation.
Well, one possible plot could be that Lucy has to face the challenges of pregnancy alone after Natsu leaves. She might struggle with financial difficulties and the emotional turmoil of raising a child without the father. She could then find support in her friends in the guild, who help her through the tough times and she eventually becomes a strong, independent mother.