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.
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.
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 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.
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.
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.
Lucy is usually one of the main female characters. She has a deep bond with Natsu through their adventures together.
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.
The OC could have accidentally said or done something that made Natsu feel unwanted or unappreciated in Fairy Tail. For example, if the OC is a new member and got more attention than Natsu in a certain situation, it might have bruised his ego and led to his decision to leave.
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 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.