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 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.
In some fanfictions where Natsu becomes END, it often leads to a power - up that is both amazing and terrifying. His physical appearance might change, with more demonic features. His power level could skyrocket, making him one of the most powerful beings in the Fairy Tail universe. He might struggle with the newfound darkness within him, as END represents a very powerful and potentially evil force. This could lead to interesting storylines where his friends in Fairy Tail have to find a way to bring back the Natsu they know while still dealing with the power of END that he now wields.
In some fanfictions, when Natsu turns into END, there's often a huge power boost. This can lead to him facing off against stronger enemies that he couldn't before. For example, he might take on the most powerful dark mages single - handedly. His appearance also changes, usually becoming more menacing with features associated with END. This transformation can also cause some internal conflict within the Fairy Tail guild, as some members may be afraid of his new form.
Natsu could develop a more empathetic side. Knowing Zeref as his brother, he might start to understand the pain and loneliness that Zeref has endured. Zeref, on the other hand, could become more open and less isolated. He might find a new purpose in protecting Natsu and the Fairy Tail guild. For example, Zeref could start to use his powers for good instead of causing destruction.
He could become more brooding. Instead of his usual carefree self, he might be burdened by the knowledge of his true nature as END.
One possible plot development could be how Natsu, as a girl, changes the dynamics within the Fairy Tail guild. Maybe she forms different relationships with the other members. For example, her friendship with Lucy might become more sisterly or there could be new romantic interests from other guild members who see her in a different light.
In the fanfiction, perhaps Natsu has a fateful encounter with a powerful magical being. This being could transfer some of its essence or powers to Natsu. For example, it might be a dragon from a different dimension that gives Natsu the ability to not only control fire on a much larger scale but also to manipulate other elements to some extent, thus making him OP. He could then use these newfound powers to take on the toughest enemies in Fairy Tail, like the dark guilds and their powerful leaders.
Sure, in the 'Fairy Tail' universe, the concept of Natsu being END is quite fascinating. Fanfiction writers often take this idea and run with it. They might create stories where Natsu discovers his true nature as END more gradually, or they could write about how the other guild members react when they find out. It can also lead to stories that explore the origin of Natsu as END and how it ties into the overall lore of the 'Fairy Tail' world.
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.
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.