Well, he could gain the ability to combine his fire magic with other elements he encounters during his travels. For example, if he comes across a powerful wind - based magic user, he might learn to merge his fire with the wind to create a more devastating and far - reaching attack. This would not only increase the power of his attacks but also give him more versatility in combat.
He might acquire enhanced fire - based powers. Maybe he learns to control the temperature of his flames more precisely, like being able to make them extremely hot for more powerful attacks or cool them down slightly to create a smokescreen for strategic escapes.
He could meet powerful mages from other guilds. These mages might have different styles of magic that Natsu can learn from.
Well, he could meet some old friends or foes. Maybe he runs into an old enemy who has also been training and they have a rematch which pushes Natsu to new limits. Or he meets an old friend who gives him some valuable advice on how to control his magic better during this trip.
He could have the power to control different types of fire, not just the regular one. For example, blue - flame fire that is much hotter and more destructive.
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.
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.
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.
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.
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.