It could mean that in the 'Fairy Tail' universe, the character Natsu has reached a point where there is an excessive amount of fan - created fictional works about him that he, in a sense, has 'had enough' of. Maybe the quality or quantity of these fanfictions has become overwhelming for the perception of the character within the fandom.
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.
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.
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 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.
One great fanfiction that delves into Natsu's memories might be titled 'Natsu's Hidden Past'. In it, the author could explore his early days in Fairy Tail, his relationship with Igneel, and how those experiences shaped the person he is today. It might show how he learned to control his dragon slayer magic and the friendships he formed along the way.