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.
One interesting natsu leaves fairy tail fanfiction could be about him going on a solo adventure to discover his true power. He might face challenges that he couldn't in the guild and grow in ways unexpected. Maybe he meets new characters who teach him different forms of magic.
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.
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 a group of rogue mages who have a vendetta against Natsu and Erza. They see the two leaving Fairy Tail as an opportunity to attack them when they are more vulnerable outside the protection of the guild.
One possible storyline could be that the OC tries to find Natsu after he leaves Fairy Tail. The OC might have a special bond with Natsu and be determined to bring him back. They could go on a journey where they face various challenges and meet new characters along the way. Another idea is that the OC is actually the reason Natsu leaves Fairy Tail in the first place. Maybe there was a misunderstanding or a conflict between them that led to his departure, and the story could be about how they resolve it.
Gray could be one. Since he and Natsu have a long - standing rivalry, Gray might try to bring Natsu back to Fairy Tail. He'd be conflicted between his annoyance with Natsu and his concern for his former guildmate.
In 'natsu leaves fairy tail harem fanfiction', a common storyline could be that Natsu leaves to seek his own power or identity outside of the guild and the harem. The female characters then form a sort of search party to find him. Along the way, they encounter different challenges that make them grow both as individuals and in their relationships with each other. This could also lead to some of them realizing that they don't just want Natsu back for the sake of the harem, but because they truly care about him as a person.
You can try websites like Fanfiction.net. It has a large collection of Fairy Tail fanfictions, and you can easily search for ones related to Natsu leaving. Archive of Our Own (AO3) is also a great place. There, you can find well - written and diverse fanfictions on this topic.