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.
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.
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.
In a fanfiction where Natsu turns into a kid, his personality would likely remain somewhat the same but be more child - like. His impulsiveness could become even more extreme as a kid. He might make new friends more easily among the younger members of the guild. And his love for food would probably stay strong, but he might be more likely to steal sweets like a typical kid. His enemies might underestimate him in this form, which could lead to some interesting battles where he uses his child - like charm and his still - powerful magic to win.
Yes, in a lot of Fairy Tail fanfiction, Natsu can be overpowered. Writers often like to amp up his powers for more exciting storylines. For example, they might give him new fire - based abilities or make his existing ones even stronger to defeat more powerful foes.
In fanfiction, Natsu's age can vary greatly. Some authors might keep him at his canonical age from the original Fairy Tail series, which is around 17 - 18 years old at the start. However, in alternate universe fanfictions, he could be older or younger depending on the plot. For example, in a time - travel based fanfic, he could be much older if the story takes place far into the future or much younger if it goes back to his childhood days.
Well, in the 'natsu has fallen protecting fairy tail fanfiction', it might show Natsu facing an overwhelming threat. Maybe it's a dark wizard or a horde of demons. He puts his life on the line to safeguard Fairy Tail, the place he loves and his friends there. His fall could be the catalyst for other characters to grow stronger, to avenge him or to carry on his legacy within the guild.
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.
One possible aspect of a Fairy Tail Natsu and Tartaros fanfiction could be Natsu's battle against the powerful foes in Tartaros. He might face new and terrifying enemies that test his limits. His dragon slayer magic would be put to the extreme, and perhaps he discovers new powers along the way.
In fanfictions like this, the character development of Natsu within the context of Sabertooth is interesting. He could learn new magic techniques from them or teach them his own unique brand of Fairy Tail spirit. It might also explore how the different cultures of the two guilds clash or blend.