In a fanfiction, Madara could be sealed within an ancient and mysterious object. For example, a long - forgotten scroll that has the power to contain his immense power. Naruto might discover this scroll during his adventures and use it to seal Madara away. This could lead to a whole new arc in the story where Naruto has to protect the scroll from those who would try to release Madara.
One possible fanfiction could be where Madara is sealed during the Fourth Great Ninja War in a different way. Maybe instead of being sealed by Hagoromo's chakra, he is sealed by a powerful new jutsu created by Naruto and Sasuke together. This would change the power dynamics in the ninja world significantly as his threat would be eliminated in a more unique manner.
There could be a fanfiction where Madara is sealed within the mind of Naruto. Since Naruto has such a strong will, he could use his own mental strength to keep Madara trapped inside him. This would create an internal struggle for Naruto as he has to constantly fight off Madara's influence while also using Madara's knowledge and power to protect the village and his friends.
In some Naruto fanfictions, Madara might be sealed through a powerful jutsu created by the main character. For example, the protagonist could combine different elemental chakra natures to form a unique sealing technique specifically designed to counter Madara's power.
In a fanfiction regarding Madara's sealed state, it could explore the emotional side of the characters who were involved in his sealing. For example, Hashirama might have complex feelings about finally sealing his old friend. It could also show how the younger generation views this significant event in history and how it shapes their understanding of the ninja world's past and future.
There could be a fanfiction where Naruto is led to the location of Madara's sealed fan by a strange prophecy. When he finally finds it, the fan is in a state of disrepair. Naruto uses his Uzumaki knowledge of sealing to repair the fan. As he does so, he uncovers secrets about Madara that were long forgotten. This discovery then sets in motion a new series of events in the Naruto world, as different factions learn of Naruto's new possession.
In some fanfictions, it could be due to a miscalculation during a powerful jutsu battle. For example, if they were trying to seal away an even greater threat together, but something went wrong and they ended up being sealed instead. This could lead to a whole new set of adventures for the other characters in the Naruto world as they try to figure out how to free them.
In this fanfiction, a plot twist could be that the seal actually has a secondary effect. Instead of just containing Madara, it also starts to absorb some of his power into Naruto. So Naruto gradually becomes stronger in a very unorthodox way. However, this also comes with risks as the power of Madara might corrupt Naruto's mind. Also, there could be a group of ninjas who believe that Naruto should be killed to ensure Madara is never released. This would put Naruto in a difficult situation where he has to fight not only against the possible corruption within him but also against those who see him as a threat because of Madara's presence inside him.
An interesting idea is that when Madara is sealed, his chakra is spread across the ninja world. Naruto has to go on a journey to collect all these fragments of chakra to prevent Madara from ever being resurrected. Along the way, he meets different clans who have been affected by Madara's actions in the past and they either help or hinder his quest.
Naruto might first struggle to understand the power of the fan. Since it was Madara's, it could have dark and complex powers. He may have to train himself to control it, perhaps with the help of Kakashi or Jiraiya (if he were still alive).
Well, his relationship with Kakashi would change significantly. Kakashi, being Naruto's mentor, would be extremely worried about Naruto. He would constantly be on guard, watching for any signs of Madara's influence on Naruto. Naruto, on the other hand, might feel a bit distanced from Kakashi as he tries to deal with this new reality on his own. With Sakura, she might be torn between her loyalty to Naruto and her fear of what Madara could do. And his relationship with the other villages would be strained. They might see Naruto as a ticking time bomb and start to isolate Konoha because of it.
It would likely have a huge impact. Naruto is known for his positive and determined nature. Having Madara sealed in him might introduce a struggle between his own beliefs and the dark and powerful influence of Madara. He may constantly have to fight against Madara's thoughts trying to overtake him, which could make him more cautious and perhaps a bit more jaded in his view of the ninja world.
One plot idea could be that the seal on Madara contains a hidden power that starts to leak out and affect the surrounding area. This causes strange phenomena, and the characters have to figure out how to stop it without fully releasing Madara. Another idea is that a character has a vision of Madara breaking free from the seal in the future, and they embark on a journey to prevent it by strengthening the seal in the present.