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.
Another scenario could be that Natsu leaves the guild to protect it. There could be a powerful and dangerous enemy who is targeting Natsu specifically. He doesn't want the guild to get involved and be at risk, so he chooses to leave in order to draw the enemy away from Fairy Tail.
In this fanfiction, perhaps Natsu leaves the guild due to a misunderstanding. He might overhear something that makes him think his friends don't trust him anymore. So, in his typical impulsive way, he decides to leave and prove himself on his own.
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.
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.
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, 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.
One possible plot could be that Natsu and Erza leave Fairy Tail due to some kind of internal conflict within the guild. Maybe there are differences in opinions regarding how the guild should be run or what kind of jobs they should take. So they decide to set out on their own journey to find a new purpose.
It could be that Natsu has some sort of falling - out with the members of Fairy Tail. Perhaps there was a misunderstanding during a big mission, and he decides he can't stay any longer. For example, if his actions were misinterpreted and the guild members blamed him for something that wasn't his fault. He might leave to prove his innocence elsewhere.
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.
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.
Well, it could be about Naruto somehow ending up in the Fairy Tail world and meeting Natsu. Maybe they team up to fight a common enemy. Naruto's jutsus and Natsu's dragon slayer magic could combine in interesting ways during battles.
In this fanfiction, Naruto could build a guild to train the next generation of ninjas in a new way. He realizes that the traditional village - based training has some limitations. So, he creates a guild where the focus is on individual growth as well as teamwork across different bloodlines and abilities. The guild might also attract some former villains who want to reform and contribute to a new ninja society.