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.
Well, perhaps Natsu returns to find Fairy Tail in a state of chaos, with many of his friends held captive by a new and formidable enemy. He then has to use his wits and strength to free them one by one, all while facing various challenges and traps set by the enemy. In this process, he might also re - connect with some old allies who had gone into hiding or been lost for a while.
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.
It could be a rogue dragon slayer. Someone who has turned against the ways of dragon slayers and wants to destroy Fairy Tail for some personal vendetta against Natsu or the guild as a whole.
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.
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 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.
One interesting plotline could be Natsu coming back with new and powerful magic. For example, he might have learned some ancient spells during his time away. Another could be his reunions with the various members of Fairy Tail. He would have emotional moments with Lucy, Happy, and the others, reminiscing about old times and looking forward to new adventures together.
He could become more brooding. Instead of his usual carefree self, he might be burdened by the knowledge of his true nature as END.
Gray would be one of the main characters to interact with Natsu. Their friendly rivalry is well - known in Fairy Tail. Makarov, as the guild master, would also have an important role in interacting with Natsu. He might give Natsu some important tasks or just have a grandfatherly chat with him about his return.
In some fanfictions, Natsu might save Lisanna from a powerful dark wizard. He could use his Fire Dragon Slayer magic to overpower the enemy and rescue her.
In some fanfictions, Lucy might use her Celestial Spirit magic. For example, she could summon a powerful spirit like Leo to help fight off whatever is threatening Natsu. The spirit could provide a distraction or directly combat the enemy while Lucy tries to reach Natsu and get him to safety.