What are the possible character developments for Natsu and Zeref as brothers in Fairy Tail fanfiction?

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.

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.

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 possible plot twist could be that Zeref actually doesn't want to fight Natsu seriously and has some hidden agenda. Another could be that Natsu discovers a new power during the fight that is related to his past or his true identity.

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.

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 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.

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.

He could become more brooding. Instead of his usual carefree self, he might be burdened by the knowledge of his true nature as END.