One possible theme could be friendship. Since both Naruto and Natsu (from Fairy Tail) are known for their strong bonds with their friends, a fanfic could explore how characters from both series form new friendships or strengthen existing ones while on the 'trains' adventure.
Another theme could be growth. Naruto and Natsu both grow and develop throughout their respective series. In a 'natsu trains naruto fanfic', the characters might experience personal growth during their time on the train. This could be in terms of their fighting abilities, their understanding of different cultures (if there's a crossover aspect), or their emotional maturity.
Well, I'm not sure about a specific 'natsu trains naruto fanfic' off the top of my head. But you can try searching on fanfic platforms like Fanfiction.net or Archive of Our Own. There are usually a lot of Naruto fanfics there, and you might be able to find one related to 'natsu trains' if it exists.
Often, there's a theme of determination. Natsu's serious training implies that he has a strong will to achieve a certain goal, whether it's mastering a new power or preparing for a big battle. Also, the theme of growth is prevalent. As he trains seriously, he grows not just in strength but also in character, becoming more mature and responsible in some fanfictions.
Another plotline could be that there's a new villain who is a threat to both their worlds. Naruto starts training Barry so that they can team up and fight this villain. Along the way, Barry learns about Naruto's past and his struggles, which makes him more determined to succeed in the training. Naruto, in turn, is impressed by Barry's scientific knowledge and how he uses it to enhance his powers.
Love and romance are obvious themes. It could explore how their relationship developed from friendship to love. Also, themes of loyalty and support, like how they support each other in difficult times. Maybe there are themes of self - discovery too, as they grow in their relationship and as individuals.
Power comparison is a common theme. Writers often like to see how Natsu's magic stacks up against Naruto's ninja techniques.
It could be that Hera is a powerful spirit or goddess in the Naruto universe who has been awakened. She sees potential in Naruto and decides to train him to reach his full potential. The plot might include Naruto facing challenges within himself as he tries to master the new skills Hera teaches him. There could also be a sub - plot where other ninja clans become suspicious of Naruto's new powers and training, leading to some conflict.
In Naruto fanfic, scorpion trains could be some unique and creative interpretations. Maybe they are some kind of new jutsu - like a train - shaped attack made by a character who has scorpion - like abilities. For example, a character might use their chakra to form a large, scorpion - tailed train that can move at high speed and attack enemies.
One common theme could be Naruto's extreme self - discipline. In these fanfictions, he might push himself to the limits in cold weather, like running barefoot in the snow to toughen his body. Another theme could be his isolation during this training. He may choose to train alone in a cold, desolate place, away from his friends, to focus solely on improving his skills. Also, there could be a theme of self - discovery. As he endures the cold and the hardships of training, he might learn more about his inner strength and capabilities.
Perhaps there could be a plot development where other characters react to Naruto's training with Deidara. For example, Sasuke might be suspicious or even jealous of Naruto's newfound skills. This could create tension between the two characters and lead to some exciting confrontations.
One common theme is the transformation of Naruto. Through Kabuto's training, Naruto often becomes more versatile in his skills. Another theme is the exploration of Kabuto's motives. Why does he train Naruto? Is it for personal gain or some greater good?