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.
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 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.
Well, in fanfiction, the 'Naruto Sealed Hatred' form is a concept that allows for a lot of creativity. It could mean that Naruto's experiences that might have led to hatred, like the way the villagers treated him as a child, are nullified in this form. Maybe he becomes a more 'pure' version of himself. This form could also be used to set up unique storylines, such as Naruto having to face an enemy who wants to release his sealed hatred for their own evil purposes.
It could be from an ancient and forgotten clan. Maybe a clan that had the power to tame and seal dragons long ago, and for some reason, they chose Naruto as the vessel for this dragon. Another possibility is that it came from a different dimension where dragons are common, and through some magical accident or ritual, the dragon ended up sealed in Naruto.
Maybe it's an ancient evil that was sealed away long before Naruto but managed to break free during his 1000 - year imprisonment. This evil entity now wants to destroy the world and sees Naruto as an obstacle once he is unsealed.
Maybe Naruto is sealed by a powerful and ancient evil force. During the 1000 years, the world he once knew could change completely. His friends and the Hidden Leaf Village might have long been forgotten or turned into legends. When he is finally unsealed, he has to adapt to this new world and face new threats that emerged during his long absence.
One interesting Naruto sealed fanfiction could be about Naruto unlocking a special power from the seal in a unique way, like during a fight with a new and powerful enemy. This power could be related to an ancient jutsu sealed within him.
The balance of power in the Naruto universe of the fanfiction could be disrupted. If Bardock was a powerful force against some impending evil, his sealing could allow that evil to thrive. For example, if there was a powerful enemy that Bardock was meant to fight, with him sealed, the enemy could start to conquer and cause chaos more easily. Also, his friends or allies might start a quest to unseal him, which could lead to many adventures and encounters with other characters, both good and bad.