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.
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 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.
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.
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.
Lucy could be a main character. She was always close to Natsu in the original Fairy Tail. So when Natsu returns as an 'ex' and meets the guild again, she would likely play a big role in how he is received and in the development of the story.
One possible storyline could be Natsu founding Fairy Tail after a great battle where he realizes the need for a guild that stands for true friendship and strength. He then gathers like - minded mages and builds the guild from the ground up.
One possible theme could be power and responsibility. As Natsu is in a 'king' role, he has to handle the power that comes with it and the responsibilities towards his friends and the guild.
One reason could be that the fanfictions don't match his view of the Fairy Tail world. He has his own understanding of the guild, his friends, and their adventures, and when fanfictions deviate from that, he doesn't like it.
A plot point could be the couple's journey to find the perfect wedding venue. They might explore different magical places in the Fairy Tail world, like a floating castle or a hidden valley. Then there's the issue of the wedding dress. Lisanna could have a hard time choosing one, with all her friends giving different opinions. In the meantime, Natsu could be training to become a better husband, learning how to cook and do household chores with the help of some of the other guild members. And finally, the actual wedding day could be filled with unforeseen events, like a sudden storm that they have to magically overcome to have the perfect ceremony.