The 'natsu dark past secret fanfiction' might revolve around Natsu's family. There could be some dark secrets about his family that he has just discovered. Maybe his family was involved in some sort of dark magic or a feud with another powerful group, and Natsu is now starting to uncover the truth about it.
Well, it could be things like a tragic family history. Maybe Natsu's family was involved in some sort of dark magic that got them in trouble. Another possibility is that Natsu had a past encounter with a powerful and evil entity that left a mark on his psyche.
It could be someone from Natsu's past. Maybe an old rival who knows about his dark secrets and wants to use them against him.
It could be about Natsu finally opening up about a tragic event from his childhood that shaped his personality. Maybe it involves the loss of a loved one, or an experience that made him distrust a certain group of people.
Another plot twist could be that the past they remember isn't actually real. It was all an illusion created by a powerful mage. When they find out, they have to figure out who the real enemy is and why they were put in this false past. This would completely shift the focus of the fanfiction from their relationship in the 'past' to finding out the truth behind the illusion.
Well, one possible plot could be that Natsu, after some sort of betrayal in Fairy Tail, decides to create his own dark guild. He might gather some disgruntled mages who also feel wronged by the mainstream guilds. Their goal could be to change the magic world's power structure in a more radical way, using darker forms of magic that are usually shunned. For example, they could explore ancient and forbidden magic spells to gain an edge over other guilds.
A plot could be that Natsu and Lisanna have been longing for a baby. They go through a magical ritual in their guild to increase their chances of having a child. Once Lisanna becomes pregnant, the whole guild gets involved. Everyone is excited and offers their help. Natsu becomes super protective of Lisanna and the unborn baby. There could be a sub - plot where an enemy tries to target Lisanna because of the powerful baby she is carrying, and Natsu has to fight to keep them safe.
Perhaps the secret is that Lucy has a unique magical item that is very powerful and dangerous. Natsu keeps it a secret because he doesn't want other mages in Fairy Tail to get greedy or misuse it. This item could be something that was passed down in Lucy's family and she doesn't fully understand its potential yet.
Perhaps Mikey is secretly related to the antagonist in the fanfiction world. This connection might be something he's ashamed of or that could put his friends in danger if discovered. For example, his family might have a dark history with the villain, and he's worried about how others would react if they knew.
His friends could be involved. For example, Lucy might stumble upon some old records or meet someone who knew Natsu in the past. Gray could also be part of it, especially if it has something to do with the magic world's secrets that they both are a part of.
Well, perhaps Natsu could be sparring with Lucy and accidentally land a more powerful blow than he intended during a training session. Maybe he's in his dragon force mode and loses control for a second, hitting Lucy hard enough to make her fall and get injured.