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.
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.
He could leave because of a misunderstanding. Maybe he overhears something wrong and thinks the guild doesn't trust him or value him anymore. So in a fit of anger and pride, he decides to leave.
The guild would be in chaos. The members would be shocked and divided. Some might side with Natsu if they believe there was a just cause, while others would see him as a traitor. There would be a power vacuum, and different members might vie for the position of guild master. This could lead to internal strife and potential break - up of the guild.
The guild would likely be shocked at first. They consider Natsu as one of their own, so his attack would be completely unexpected. Some members might try to talk him down, like Lucy. She has a close bond with Natsu and would probably try to reason with him.
She would fit in very well. The Fairy Tail guild is all about family and acceptance. Given her parents are Lucy and Natsu, she'd be welcomed with open arms. She could learn from the older members like Makarov and Mirajane, and bond with the younger ones like Romeo. Her magic skills would also contribute to the guild's strength during battles or when helping the town.
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.
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.
The guild members' reactions would vary greatly. Happy, being Natsu's closest companion, would likely be excited about the fanfiction. He might fly around the guild hall, telling everyone about it and adding his own little comments about how accurate or inaccurate it is. He would definitely be protective of Natsu's image in the fanfiction, especially if it's not a positive portrayal.