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.
Erza is also a favorite. She can be a strong female figure in relation to the 'brothers' in the fanfiction. Maybe she acts as a mentor or a guiding force for them. Her strict yet caring nature can add depth to the story, like when she trains the 'brothers' to become better mages or helps them resolve their internal conflicts.
One idea could be exploring the backstory of the brothers before they joined Fairy Tail. Maybe they had a rough childhood in a different town and had to rely on each other to survive. They could have had a special encounter with a powerful mage that inspired them to become stronger and seek out Fairy Tail.
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 interesting plot could be that the Dragneel brothers bring new magic techniques to Fairy Tail. For example, Natsu might introduce a lost form of dragon slayer magic that he discovered during his travels. This could lead to new training regimes for the other wizards in the guild. Another plot could involve the brothers' past. They could uncover secrets about their family that are somehow linked to the founding of Fairy Tail, creating a whole new backstory for the guild.
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.
Natsu's hot - headed and impulsive personality could bring a lot of energy to Fairy Tail. He'd be the first to jump into action during a crisis, inspiring others to be brave. Zeref, on the other hand, if he's redeemed in the fanfiction, could bring a sense of mystery and wisdom. His calmness could balance out Natsu's exuberance, creating a more diverse dynamic within the guild.
It adds more content. The official Fairy Tail series has a limited number of episodes and chapters. Fanfiction fills in the gaps. For example, it can create stories about what the characters were doing during the time - jumps in the original story.
If Lucy never joined Fairy Tail, the friendships and relationships among the characters would form in a very different way. Also, many of the story arcs might not unfold as they do in the original plot.