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.
He might gain some of Madara's Sharingan - related abilities. Maybe he could use some of the advanced genjutsu techniques that Madara was proficient in. Since Madara had strong chakra, Naruto could also potentially have an increase in his own chakra levels or a new way to manipulate chakra, like Madara's ability to control the Susanoo.
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.
I'm not entirely sure if there is a specific fanfiction with exactly that premise. However, given the popularity of both Naruto and the concept of sealing powerful entities in Naruto's world, it's quite possible. Fanfiction writers love to experiment with combining different fictional elements, so it wouldn't be surprising if someone has written a story where Jenova is sealed in Naruto. You can try searching on popular fanfiction websites like Fanfiction.net or Archive of Our Own to find out for sure.
In many such fanfictions, it could be a variety of deities. It might be an ancient and powerful god from Naruto's world's mythology that the story's author has chosen to insert. For example, it could be a god of elements like a super - powerful fire god, which could give Naruto even more control over his existing fire - based jutsu and new, more extreme abilities related to fire.
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.
With his enemies, it would be a whole different story. They might be more cautious about attacking Naruto. Some might even try to find a way to use the god sealed in him for their own purposes. And Naruto's relationship with the hidden villages could also change. They might see him as a potential threat or a very valuable asset.
Well, Naruto might become more mysterious. Since Gaster is an unknown and rather enigmatic entity, Naruto could start having strange thoughts or powers that he doesn't fully understand at first. He might seem more withdrawn as he tries to figure out what's going on inside him, and his normal cheerful and straightforward personality could be clouded by the presence of Gaster.
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.
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.