It could be a rogue ninja who has been secretly observing Naruto during his training trip. This ninja, jealous of Naruto's growth, decides to target him and the village he protects. He has some unique and dangerous abilities that even Naruto's new - found power will have a hard time dealing with at first.
Perhaps an old enemy of the Leaf Village, who was thought to be defeated long ago, has returned even stronger after Naruto's training trip. This enemy has a personal vendetta against Naruto and the village. He has been biding his time, gathering strength and allies, and now that Naruto is back and seemingly more powerful, he decides it's the perfect time to strike and show that he is still a force to be reckoned with in the ninja world.
One possible plot could be that Naruto returns from the training trip with new and extremely powerful jutsu. He surprises everyone in the village, including his friends and rivals. He then uses his newfound strength to take on tougher missions and protect the village from a new and powerful enemy that was previously too strong for the village to handle.
He could meet an old hermit ninja who imparts his wisdom and knowledge to Naruto. This hermit might have been living in seclusion for years, but seeing Naruto's potential, decides to teach him things not found in normal ninja training.
There's also the possibility of a new, unknown threat emerging from a different dimension or realm. This antagonist could be a powerful being with abilities that the ninja world has never seen before. He sees Naruto's newfound power as a threat to his own plans of conquest and decides to target him first, leading to a series of epic battles across different locations in the Naruto universe.
In such a fanfiction, Naruto might return to the village with new and powerful jutsu. He could surprise his friends with his enhanced skills. For example, he might be able to create new types of shadow clones or have a better control over his chakra. Maybe he would also have new insights on how to deal with his enemies, like Orochimaru or the Akatsuki.
He could meet some old enemies who have changed their ways. For instance, some of Orochimaru's former minions might have decided to start anew and cross paths with Naruto. Or perhaps he meets a hermit ninja who has been living in isolation for years and imparts some valuable wisdom to him.
In the fanfiction's training trip, Naruto may encounter some spiritual beings. These beings could be related to the origin of chakra or some long - forgotten ninja legends. Interacting with them would be a very different experience for Naruto. He would have to use not only his physical skills but also his wits and understanding of the ninja world's deeper mysteries to communicate with them. This could lead to a major transformation in his ninja journey, as he delves deeper into the spiritual side of the ninja arts.
It could be a rogue ninja who has a special ability to disrupt shadow clones. This antagonist might be jealous of Naruto's ability to use shadow clones for training and wants to stop him. Maybe he has a jutsu that can make Naruto's clones go berserk and attack him instead of following his commands.
Naruto might meet some foreign ninjas from far - off lands that have completely different fighting styles. These ninjas could teach him new ways to use his chakra and improve his combat abilities. He may also encounter some mysterious hermits living in the mountains or forests who possess great wisdom and unique jutsu. They could guide him on his journey of self - improvement and give him valuable advice on how to become stronger as a ninja.
One possible plot could be that Naruto returns from his training trip with new jutsu and a more strategic mind. He starts to outwit his opponents in battles, surprising everyone in the village. For example, he uses his newfound knowledge to defeat a group of rogue ninjas who were causing trouble near Konoha.
The main characters are Naruto and Jiraiya. Naruto is the young and eager - to - learn ninja, and Jiraiya is his experienced mentor.