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.
Happy is almost always there. He's Natsu's loyal sidekick and provides comic relief as well as support during training. Lucy also frequently appears. She might offer some strategic advice or just be there to cheer Natsu on. Erza is another common character. Given her strong and strict nature, she could be the one pushing Natsu to his limits during training.
One common theme is Natsu's growth in power. In these fanfictions, he often overcomes challenges during training that make him stronger for future battles. Another theme could be his relationships. Training might strengthen his bonds with his guildmates as they support or train with him. For example, his friendship with Gray could develop as they spar during training sessions.
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.
One popular natsu training trip fanfiction might be 'Natsu's Epic Training Adventure'. It could detail his growth in strength and new magical abilities he discovers during the trip.
We could expect to see Natsu encounter different magical creatures and environments during his 10 - year trip. He could be in a land full of ice - based magic and have to adapt his fire magic to survive. This would lead to some really interesting battles and power - up moments for him. And of course, he might come across some lost or hidden magic spells that he could bring back to Fairy Tail.
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.