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.
I haven't read this particular fanfiction yet. But generally, in a scenario where Fairy Tail characters watch Natsu fight Zeref, it could be filled with intense emotions. Natsu's determination and his unique powers would be on full display. And the reactions of the other Fairy Tail members would add depth to the story, whether it's shock, pride, or worry.
If Natsu and Zeref join Fairy Tail in fanfiction, it would change the dynamic of the guild completely. The other members would have mixed reactions. Some might be distrustful of Zeref due to his past actions. However, as time passes, they could see the good in him. Natsu and Zeref together could attract a lot of attention from outside forces. Enemies might target Fairy Tail more because of their combined power. But also, they could inspire the guild to become even stronger, with everyone striving to keep up with their new powerful comrades.
In some fanfictions, Zeref is portrayed as a more sympathetic character. Instead of just being the big bad, he might be shown as a victim of his own power or circumstances. For example, he could be a character who was forced into doing evil things and actually has a good heart deep down.
Some common themes are redemption and forgiveness. Given Zeref's actions in the original Fairy Tail series, fanfictions might delve into whether Natsu could be the one to bring about Zeref's redemption. Family also plays a role. With their shared history, fan stories could explore the idea of them being part of an unexpected family or having a brotherly bond in a different way than what's shown in the official story.
Perhaps a plotline where an outside force tries to manipulate Natsu and Zeref against each other while they are in Fairy Tail. This could lead to a lot of internal conflict within the guild as members take sides. But in the end, through the power of friendship and the values of Fairy Tail, they see through the ruse and become stronger together, uniting the guild even more firmly.
One plot idea could be that Zeref discovers a way to reverse his curse through Natsu. Since Natsu is his creation, there might be some hidden connection that could be explored. Maybe Zeref tries to use Natsu's unique dragon - slayer magic to break the chains of his dark magic.
Their relationship could develop differently if Natsu didn't initially see Zeref as an enemy. Instead of the typical animosity, Natsu could approach Zeref with curiosity. Maybe he wants to understand why Zeref created him and what his purpose really is. This could lead to a more cooperative relationship from the start.