Well, perhaps in the fanfiction, Natsu's nightmare is about not being able to use his fire magic anymore. He could be in a world where he's powerless against his enemies, like the dark guilds. When he wakes up, this might make him more determined to master his magic and become even stronger, fearing that such a situation could become real.
One possible plot could be that Natsu's nightmare is related to losing his friends. In the nightmare, he might see Fairy Tail being destroyed and his friends being hurt or disappearing. This could lead to him waking up in a panic and then being extra protective of his guildmates in the real world.
In this fanfiction, the nightmare could be centered around Igneel. Natsu might dream that Igneel is in danger or has been killed by some powerful foe. This would be a very traumatic nightmare for Natsu. As a result, he could start a journey in his dream to save Igneel, and when he wakes up, he might be more eager to find Igneel in the real world and be haunted by the fear of losing him.
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.
Well, the fanfiction could start with Natsu finding out he has a daughter he didn't know about. Maybe she was the result of some long - lost adventure or a situation he was in and forgot due to a spell or something. His daughter could have a unique blend of magic, not just fire - dragon slayer magic. She could also have some other latent abilities. The story could then focus on Natsu trying to bond with his daughter, teaching her about the values of Fairy Tail, like friendship and loyalty, while they also face threats from old enemies who see the daughter as a new target.
It could be about Natsu having left Fairy Tail for some reason and then reuniting with the guild in a very unexpected way. Maybe he has new powers or a different outlook on life that causes some friction and also some amazing moments of reunion and growth within the guild.
Well, one possibility is that Natsu could join Fairy Tail in a different way than in the original story. Maybe he has some special mission before officially becoming a member. For example, he has to prove his loyalty by helping an elder member with a personal problem.
One possibility could be that Natsu is attacked due to a misunderstanding within the Fairy Tail guild. Maybe a new member misinterprets Natsu's actions as a threat and attacks him in self - defense.
By showing his vulnerability in the nightmare. Natsu is often seen as strong and fearless. But in a nightmare, he can be scared. This contrast can reveal his deeper emotions and fears that he usually hides.
A prevalent theme is the battle against evil versions of his friends or himself. In a nightmare fanfic, the author might create a scenario where Natsu has to fight an evil Natsu or his friends who have been corrupted. This theme adds a lot of drama and action. It also tests Natsu's loyalty and his ability to see the good in his friends even when they seem to be his enemies.
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.