Another possibility is that Natsu is forced to leave Fairy Tail forever. There could be an outside force, like a powerful dark guild or a magic - user with a vendetta against Fairy Tail. They might target Natsu and make him a liability to the guild. In order to protect his friends and the guild he loves, he decides to leave so that the danger doesn't follow him back to Fairy Tail.
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.
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 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.
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.
One possible plot development is that Natsu, as END, could be hunted by various magical organizations who see him as a threat. This would put him on the run and force his Fairy Tail friends to either protect him or try to stop him if they believe he's too dangerous. Another development could be that Natsu discovers new abilities related to END that give him an edge in battles but also come with a cost, like draining his life force or affecting his mental state.
One possible reason could be a misunderstanding within the guild. Maybe someone spreads false rumors about Natsu, leading others to believe he's done something wrong. Another reason might be a power struggle. If there are those in Fairy Tail who envy Natsu's strength and popularity, they could plot against him to get him out of the way.
One possible plot development could be that Natsu discovers a new and powerful magic during his return that gives him an edge in saving Fairy Tail. Maybe he finds a way to combine his Fire Dragon Slayer magic with some ancient magic he stumbles upon on his journey back.
One possible plot could be that Natsu's nightmare is related to losing his friends. In the nightmare, he might see Fairy Tail being destroyed and his friends being hurt or disappearing. This could lead to him waking up in a panic and then being extra protective of his guildmates in the real world.
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 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.