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.
Perhaps it's a jutsu related to his connection with the Nine - Tails. He might develop a jutsu that allows him to fully utilize the Nine - Tails' chakra in a more controlled and strategic way. For example, he could create a shield made of Nine - Tails' chakra that can not only protect him but also absorb the opponent's attacks and convert it into his own energy.
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.
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.
Well, in the Naruto universe, for Naruto to make a Treant Jutsu, he might first need to study nature chakra deeply. He could observe how trees grow and thrive, and then try to infuse his own chakra with the essence of nature. Maybe he'd have to combine his shadow clone jutsu in a new way, having clones study different aspects of trees, like their root systems and how they absorb nutrients. Then he could use his Rasengan as a base and add the nature - based tree - like elements to it, gradually forming a Treant - like structure out of his chakra.
Well, Naruto could use the Smoke Jutsu to create illusions within the smoke. He might make it seem like there are a hundred Narutos emerging from the smoke, scaring his enemies. This would be a creative use of the basic Smoke Jutsu, adding an element of deception.
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.
Maybe some would try to use it as a way to show female empowerment in the Naruto world, but it's a very strange and inappropriate way to do so. The Naruto series has its own ways of showing strong female characters without having to rely on such fanfic themes. For example, characters like Tsunade are shown as powerful leaders and ninjas in the official story.
He could potentially learn the Flying Thunder God Technique. It's a very powerful jutsu that Minato was famous for. Naruto's already fast with his Sage Mode and Nine - Tails Chakra, and this would make him even more formidable in battle.
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.
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.