Well, it depends on how the author develops it. Maybe it causes the OC to struggle with inner demons or makes them a target for certain enemies, creating lots of drama and action in the story.
The curse seal for the OC in such a fanfiction often becomes a central element. It could shape the OC's personality, influence their choices, and lead to intense conflicts and resolutions throughout the narrative.
The curse seal often gives characters increased power. For example, they might become stronger physically, able to take on tougher opponents. In some fanfiction, it can also change their appearance, like making their eyes glow or their skin take on a different hue.
Naruto's relationship with Sasuke could be really affected. Sasuke, having his own experiences with dark powers, might see Naruto in a new light. They could either become more competitive as they both struggle with different forms of dark power, or they could bond over their shared experiences of having something that sets them apart from others.
The curse seal can be a catalyst for Naruto's character development. It could force him to face his inner demons. Since Naruto is known for his positive attitude, the curse seal might introduce a darker side of him that he has to learn to control. This internal struggle can make his character more complex.
She could change the power balance. If she's a powerful seal master, she might be able to create seals that can counter powerful jutsus of the villains. For instance, if there's a new villain with a really strong attack jutsu, she could develop a seal to nullify it.
In fanfiction, Naruto might remove the curse seal through intense training. He could focus his chakra in a new way to counteract the dark energy of the curse seal. For example, he might use Sage Mode training to purify his chakra system and gradually force out the curse seal's influence.
Their friends might be initially afraid of them. Since the curse mark is often associated with dark power, friends like Sakura and Kakashi could be worried that Naruto and Sasuke might lose control. This could create some distance between them as their friends are not sure how to approach them anymore.
In some fanfictions, the curse mark can initially drive a wedge between Naruto and Sasuke. Sasuke might become more withdrawn and brooding due to the power and dark influence of the curse mark. Naruto, on the other hand, would be worried about his friend but might not fully understand what Sasuke is going through. This could lead to some misunderstandings and arguments between them as Naruto tries to reach out while Sasuke pushes him away.
It could derail the plot completely. For instance, if the Seal Master was supposed to teach Naruto a crucial sealing technique to stop an upcoming evil, his ignorance means that evil might succeed. So the whole story takes a darker turn as the heroes are now at a disadvantage.
In terms of his romantic relationships, if a love interest discovers the seal on his heart, it could either draw them closer or create distance. If they are understanding and want to help, it could strengthen their bond. But if they are afraid of the unknown associated with the seal, they might pull away. For example, Hinata might be initially scared but then resolve to stand by Naruto and help him understand the seal.
It'll likely change the power balance. If Naruto breaks the seal early in the fanfiction, the story could take a completely different turn. Enemies that were once a threat might become easy to defeat, or new, more powerful enemies could emerge because of the change in Naruto's power level.