Well, it depends on the specific fanfiction. Some fanfictions do make Zeref and Natsu brothers. This can create some really cool plot twists. For instance, it could change how Natsu views his powers if he discovers he's related to Zeref. And it might also make Zeref more protective or possessive of Natsu in a different way than in the original story. It's a concept that allows for a lot of creativity in the fanfiction world.
One common theme is the exploration of their shared past. Since they are brothers in these fanfictions, writers often dig into what their childhood might have been like.
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 idea could be exploring their childhood before they were separated. Maybe Zeref was protective of Natsu and taught him some basic magic skills in secret. Another could be a story where Natsu discovers his relation to Zeref during a really tough battle and has to deal with the emotional turmoil of it. For example, he might hesitate to fight Zeref as fiercely when he realizes they are brothers.
Another idea for this fanfiction could be that they were both students at a very old and secret magic academy in the past. Zeref was an outcast because of her dark magic, but Natsu didn't care. He befriended her, and together they explored the secrets of the academy and the magic within it. However, their friendship was soon tested when the academy's elders discovered Zeref's true nature.
Well, perhaps Zeref kidnaps Natsu because he sees Natsu as a key to some grand plan. Maybe he wants to use Natsu's power for his own dark purposes. For example, Zeref might think Natsu's unique magic can help him break some sort of ancient curse or gain access to a powerful magic source.
Natsu and fem Zeref are the main characters. Natsu, who is always ready for a fight and loves his friends deeply. In this fanfiction, his interactions with fem Zeref would be crucial. Fem Zeref, she could be portrayed in different ways. Maybe she is more empathetic than the original Zeref or has different magical abilities. Their combined presence drives the story forward, as the fanfiction would likely explore how they interact with each other and the world around them.
Well, it depends on the specific fanfiction. Some might have a plot centered around their unique friendship in a different AU (alternate universe), like they team up to fight a common enemy. Others could explore their relationship in a more emotional way, perhaps with Zeref having to deal with her new female form and Natsu being there to support her in ways different from the original story.
Well, one possible plot could be that Zeref kidnaps Natsu in an attempt to use his power for some dark purpose. Maybe he wants to turn Natsu into a weapon against Fairy Tail. Zeref might believe that Natsu's unique dragon - slayer magic can be harnessed in a way that no one else's can.
A lot of these fanfictions also have a theme of redemption. Fem Zeref might be portrayed as a character seeking redemption for past actions, and Natsu could be the catalyst for that change. It's a way to give a new perspective to Zeref's character. Additionally, themes of friendship or unrequited love are also common. The relationship between Natsu and Fem Zeref can be explored in a platonic or romantic way, and often there are misunderstandings and moments of self - discovery related to these feelings.