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.
Erza would be very concerned. As one of the strongest members of the guild, she would see Natsu's departure as a big blow to Fairy Tail. She might try to find out the real reason behind his leaving and see if there's anything she can do to bring him back. She would also worry about how the rest of the guild will take it, especially the younger members who look up to Natsu.
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.
Some members might be sad at first. Lucy, for example, would surely miss him a great deal as they are close friends. She might worry about him constantly during his absence.
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.
There could be a mysterious old wizard who takes Natsu under his wing when he leaves Fairy Tail. This wizard might have knowledge of ancient magics that Natsu is eager to learn.
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.
Lucy would be completely devastated. She and Natsu have a very close bond. She might go into a deep depression, constantly blaming herself for not being able to save him. Gray would be angry at first, feeling like he should have been there to help Natsu. Then he'd be sad and would probably vow to avenge Natsu. Erza would be strong on the outside but inside she'd be broken. She might try to keep Fairy Tail together in Natsu's memory. Happy would be inconsolable, constantly flying around looking for Natsu and refusing to believe he's really gone.
Lucy would be extremely distraught. She and Natsu have a very close bond. She might cry for days and even isolate herself from the rest of the guild at first. Gray would be angry and feel a strong sense of loss. He'd probably blame himself for not being there to help. Erza would be stoic on the outside but inside, she'd be deeply affected. She'd take charge of the situation, making sure Natsu's death was not in vain and that the guild stays strong.
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.