Maybe Sasuke. Although they are rivals, they also have a deep bond. Sasuke might have come across a unique jutsu during his travels and decide to teach it to Naruto so that they can both become stronger and better protect their village.
It could be a new form of Rasengan. Maybe Naruto discovers a way to combine different elements with the Rasengan more effectively, like adding lightning to it for a more powerful and faster attack.
If Naruto has a new secret jutsu in fanfic, like a super - powerful healing jutsu, it could change the story by allowing him to save more lives during battles. For example, he could heal injured comrades immediately and keep them in the fight. This would boost the morale of his side and potentially change the outcome of many battles.
One interesting Naruto secret jutsu in fanfiction could be a time - traveling jutsu. It allows Naruto to go back in time and change certain events in the ninja world. This can lead to all kinds of new storylines and character developments.
There could be a jutsu that allows for instant teleportation across long distances. This jutsu might be a new discovery in the fanfiction world, perhaps created by a character who has been experimenting with space - time ninjutsu. They might use it to quickly get in and out of battles or reach far - off places in a blink.
A third idea for a Naruto poison jutsu fanfic could be a story set in a different era of the Naruto universe. In this time, poison jutsu is a highly respected and feared art. A young ninja, inspired by tales of Naruto, tries to master poison jutsu. However, they face many challenges, including a corrupt master who wants to use the ninja's skills for evil. The young ninja must then decide whether to follow the master or break away and use their skills for the greater good.
Maybe he found some hidden scrolls left by his mother. These scrolls could have contained the details of her jutsu. Naruto, being the curious and determined ninja he is, would have studied these scrolls carefully and learned the jutsu.
He might learn a time - traveling jutsu. This would be really cool as it could allow him to go back and change certain events in the Naruto world, or gain knowledge from the past to use in the present battles.
If he learned a healing jutsu, he would become more nurturing. He could use this jutsu not only to heal his friends in battle but also to help the villagers. His reputation would change from just a powerful fighter to also a healer, and he would be more respected in a different way.
Well, in many such fanfictions, Naruto might start learning his father's jutsu when he discovers some hidden scrolls or when a mentor who knew his father decides to teach him. Maybe it could be someone like Jiraiya who reveals some secrets about his father's techniques.