Well, if Naruto is an ancient demon in fanfiction, it could lead to a change in his power dynamics. He might have more uncontrollable and dangerous powers. His place in the ninja world could be completely redefined. People might either fear him or try to use him for their own gains. His moral compass could also be affected as the nature of being a demon might bring out more primal instincts. And his relationships with characters like Sasuke and Sakura would likely be put to the test as they deal with this new version of Naruto.
Perhaps Naruto's relationship with the other characters changes. Some might be wary of him now that he has a connection with the Juubi. On the other hand, some might see him as an even greater hero for surviving with the help of such a powerful entity. Also, Naruto might have a new sense of purpose. He could feel obligated to use the newfound power or knowledge from the Juubi to protect the village in a different way, or to seek a greater understanding of the Juubi's origin and the secrets it holds.
In some fanfictions, Kaguya being sealed might lead to new mysteries emerging. For example, there could be hidden secrets or powers that were associated with her that now start to surface. This could draw in new characters or lead to new adventures for the existing ones. Maybe some of Kaguya's leftover chakra starts to influence certain events or people in unexpected ways.
One consequence could be that Naruto has some strange side - effects. Since Orochimaru's methods are often unorthodox, Naruto might have trouble controlling his chakra properly. For example, his chakra could be tainted with some of Orochimaru's dark chakra, making his jutsu less stable.
In terms of the story's plot, it could lead to Naruto trying to redeem himself in Hinata's eyes. This could involve him going on dangerous quests or changing his ways. It also might make Naruto more isolated as he loses one of the people who used to care about him, which could in turn lead to a change in his relationships with other characters like Sasuke or Sakura.
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.
The most obvious consequence would be the end of their iconic friendship. It would change the whole dynamic of the story. For Sasuke, he might feel lost or even a bit angry that Naruto, who was always there for him, is now indifferent.
One consequence could be a change in his appearance. Maybe he gets some demonic features like red eyes or strange markings. Another is that his powers might become unstable as he tries to control the demon - related abilities.
One consequence could be a change in Naruto's identity. He might have a different view of himself and his place in the ninja world. Another is that his relationship with the other characters, like Sasuke or Sakura, could be altered as they might view him differently. Also, his power origin might be more directly related to Kurama, and he could have access to different forms or abilities earlier.
Naruto might become more determined. He's the type who turns negative things into positive motivation. So, Sakura's meanness could make him strive harder to prove himself.
One consequence could be that Naruto becomes extremely powerful. He might be able to defeat enemies with ease that he previously struggled against. For example, in some fanfictions, he can single - handedly take on powerful villains like Madara or Kaguya with newfound abilities.