Maybe the fanfiction could start with Natsu waking up not knowing it's his birthday. As he goes about his day in the guild, he starts to notice little things. People are acting a bit strange, whispering and giggling. Then suddenly, they all shout 'Happy Birthday' and the party begins. There could be a friendly sparring match as part of the celebration, with Natsu showing off his new moves he's been working on. And of course, there would be lots of fire - Natsu's signature - in the form of some cool firework - like spells created just for the occasion.
Natsu could end up teaming up with some unexpected allies from Tartaros. Maybe there are some members who have a change of heart and decide to help Natsu fight against the more evil elements within their own organization.
She could be a key figure in an ancient prophecy. For example, the prophecy might state that only the son of a certain dragon - slayer mother (Natsu's mother) would be able to defeat a great evil that threatens the Fairy Tail world. This would set Natsu on a path to discover his mother's identity and fulfill the prophecy.
Natsu could also be deeply hurt. He values his friends highly in Fairy Tail. If betrayed, he might isolate himself, doubting his own judgment in trusting people. This could lead to a period of self - discovery where he tries to figure out how to trust again. For example, he might recall the times when his friends in Fairy Tail were truly there for him and use those memories to heal and grow stronger in the face of this betrayal.
In such a fanfiction, Natsu might have become much stronger. Maybe he has mastered new fire - based magic techniques. He could also have new relationships within the guild, like closer friendships or even a romantic interest.
Well, in fanfiction where Natsu leaves the guild, he could become a sort of lone wolf. He might travel to far - off lands to discover more about his dragon - slayer powers or his past. This could lead to him meeting new and powerful allies or enemies. Without the support of the guild, he would have to rely solely on his own strength and wits. It could also make him a more serious and hardened character, as he no longer has the safety net of his Fairy Tail family.
Natsu could bring a more fiery and passionate leadership style. He'd probably be more action - oriented than the previous masters. For example, if there's a conflict with another guild, he'd be the first one to charge into battle, leading by example.
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 this kind of fanfiction, the kid might have some unique abilities. Since Natsu is a dragon slayer, the kid could have an enhanced form of dragon slayer magic or a combination of different magics. For example, the kid could have the ability to not only use fire like Natsu but also manipulate wind to make the fire more powerful.
Well, he could meet some old friends or foes. Maybe he runs into an old enemy who has also been training and they have a rematch which pushes Natsu to new limits. Or he meets an old friend who gives him some valuable advice on how to control his magic better during this trip.
Well, if Natsu asks Lucy out in fanfiction, it could change the whole dynamic of the Fairy Tail group. Their friends would probably be super supportive, especially Happy. They could start exploring new places not just for quests but also for romantic getaways. And Lucy might write a new story about their relationship in her diary.