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.
Most likely, the other members of Fairy Tail would be involved. For example, Lucy might be there. She could be in distress during the attack that Natsu is trying to fend off, or she could be one of the first to find Natsu after he has fallen. Gray could also be involved, perhaps he was fighting alongside Natsu or arrives shortly after to see what has happened. Erza too, as a strong and important member of the guild, she may play a role in the events following Natsu's fall.
There are also scenarios where Lucy is targeted because of her celestial spirit magic. Natsu, being aware of the power and danger associated with it, ensures that no one can use Lucy's magic against her or the guild. He'll stand between her and any magic - user who tries to manipulate or harm her because of her unique abilities.
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.
In this kind of fanfiction, Natsu may also face challenges related to misinformation or deception. Enemies could trick him into believing that Lucy is safe when she's actually in danger, or vice versa. His emotions for Lucy can sometimes cloud his judgment as well. He might be so eager to protect her that he rushes into a situation without fully thinking it through. Additionally, there could be internal guild issues. For example, if some members of Fairy Tail have different ideas about how to protect Lucy or if there are rivalries within the guild that affect Natsu's ability to focus on protecting her. All these things can make it really hard for him to safeguard Lucy effectively.
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.