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.
In some fanfictions, Kushina was sealed into Naruto through a complex jutsu. It might be a modified version of the sealing jutsu used to seal the Nine - Tails. The process could involve special incantations and chakra manipulations.
One possible reason could be that his godlike powers were seen as a threat. The village elders or some powerful forces might have feared that Naruto, with his immense strength, could lose control and cause destruction. So, they decided to seal him away to protect the village and the world at large.
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.
Well, in some Naruto fanfictions, Bardock might be sealed by powerful jutsu. Maybe a dark ninja uses a forbidden sealing technique to lock him away, preventing him from interfering with their evil plans.
Well, one reason could be that the author wants to explore an alternate universe. Maybe in this fanfiction's version of the Naruto world, the events leading to the sealing of the Kyuubi never occurred. It could be due to different political situations among the ninja villages, or perhaps a different chain of events that prevented the Kyuubi's attack on Konoha in the first place. So, without the attack, there was no need for the seal.
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.
I'm not entirely sure as it could be a very specific fan - made story. It might be about the legacy that Naruto leaves behind which has been sealed in some way, perhaps a power or a memory. Maybe it involves Naruto's influence on the next generation and how that is locked away or preserved in the form of a sealed legacy.
It could be about many things. It might be a fan - made story that focuses on the legacy left behind by Naruto after he was sealed away. Maybe it explores how the ninja world copes without him, or perhaps it delves into the secrets and powers that were sealed along with Naruto.
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.