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.
She could also be a catalyst for character development. If she teams up with Naruto or other characters, she can teach them about seals, which not only improves their fighting abilities but also their understanding of the ninja world. This growth in knowledge can lead to changes in their personalities and how they approach different situations.
In terms of character design, as a seal master, she would likely have a unique appearance. Maybe she has tattoos or markings related to sealing on her body. Her personality could be very studious and detail - oriented, given the complexity of seals. She would also need to be strong - willed, as dealing with the power of seals requires a great deal of self - control. Her presence in the fanfiction could attract other characters interested in seals, creating a community within the story.
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.
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.
Perhaps the OC Seal Master is an outcast in their village because of their unique seal powers. They then get involved in a plot where they have to prove their worth by using their skills to protect the village from a mysterious threat. For example, an unknown force is causing seals all over the village to malfunction and it's up to the OC to figure out what's going on and fix it using their in - depth understanding of seal mechanics.
An immortal OC can also impact the plot by being a source of information. Having lived through so much, they know the secrets of the past, including things about the origin of chakra or the true nature of certain clans. This knowledge can be a driving force for the main characters' quests and can completely change the narrative as they uncover things that were previously hidden.
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.
It could lead to some really interesting plot twists. Naruto's new ability as a seal master might give him an edge in battles and change the dynamics of his relationships with other characters.
It can drive the plot by introducing new challenges. If the divine language is difficult to understand, characters may have to search for ancient texts or seek out mentors to decipher it. This search could lead them to new locations and encounters.
Naruto's life would be very different. He would be more respected in the ninja world for his unique skills. Instead of just being known for his Nine - Tails chakra, his sealing abilities would also be a major factor. He could use his sealing skills to protect the village in new ways, like creating powerful seals around Konoha to prevent enemy attacks. Also, he might be called upon more often to deal with dangerous artifacts or threats that require sealing.
A powerful and rogue ninja who has also mastered some dark seal arts could be a great antagonist. He might be jealous of the OC Seal Master's natural talent and pure intentions with seals, and tries to steal their techniques or stop them from achieving their goals. For example, he could be creating cursed seals that are causing chaos in the ninja world.