There could also be a plot where the kid's existence is threatened by an old enemy of Natsu. This enemy might see the kid as a way to get revenge on Natsu. So Natsu has to not only protect the kid but also teach the kid how to defend himself. The relationship between Natsu and the kid would grow stronger as they face these challenges together, and the kid could learn valuable lessons about courage and friendship from Natsu and the other Fairy Tail members.
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.
The story could revolve around Natsu's struggle to balance his adventurous life in Fairy Tail with being a father. He might be hesitant at first because he's so used to being on his own and going on dangerous missions. But as he spends more time with the kid, he realizes how important it is to be there for him. The other members of Fairy Tail could also play important roles, like helping Natsu raise the kid or teaching the kid their own unique magics.
Another possibility is that the kid is from an alternate dimension where time passes differently. Natsu's kid shows up in the Fairy Tail world and he has to figure out how to be a father. The story could focus on their journey to get to know each other and the kid's adjustment to this new world.
Natsu might get into a friendly competition with the new kid. For example, they could have a race around the school campus using their magic. Natsu would be all fired up to show his skills, but the new kid might surprise him with some unexpected magical abilities.
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.
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.
Perhaps the Fairy Tail members watching will have different reactions. Some might be worried for Natsu, especially those who are close to him like Lucy or Happy. Others, like Erza, might be more focused on analyzing Natsu's moves and Zero's counter - attacks. And there could be some comic relief from the likes of Gray, making snarky comments while still being engaged in the fight between Natsu and Zero.
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.
One possible scenario could be that the new kid is a huge fan of Natsu and joins Fairy Tail School Academy hoping to meet him. The new kid might be really nervous at first, but then Natsu's warm and friendly personality makes him feel welcome. Natsu could show the new kid around the school, introducing him to all the cool magical things there.
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.