I'm not sure exactly as there could be many different interpretations. It might be about new types of seals in the Naruto universe and how they affect the fate of the characters. Maybe it focuses on a character who discovers a powerful seal that changes the course of their life or the entire ninja world.
I'm not sure since I haven't read it. It could be Naruto himself, as he often has to deal with different seals and his own fate. Or it could be a new original character created specifically for this fanfic who has a special connection to the seals of fate.
There is a possibility that it was an ex - shinobi who had retired from active duty. This person had amassed a collection of seals during their career and now made a living by selling them to other ninjas like Naruto.
Obviously, Naruto is one of the main characters as the title implies he is the one sealing the Kyuubi. Also, the Kyuubi itself is a main character in a sense. It has a major role in the story as it is the entity being sealed.
Some interesting concepts include the use of seals for teleportation. In fanfic, they might create a seal that can transport a person to any location they've previously marked, without the need for complex hand signs like in the original Naruto. There could also be seals that are tied to a character's emotions or memories. For example, a seal that activates when a character feels extreme anger and gives them a boost in power. And then there are the so - called 'forbidden seals' in fanfic which often hold great power but come with a huge cost, like sacrificing a part of one's life force.
Well, perhaps in the fanfic, Naruto has to team up with his friends in a unique way to seal the Juubi. For example, Sasuke could use his Sharingan to help Naruto locate the weak points of the Juubi while Naruto does the actual sealing. And Sakura could use her medical ninjutsu to keep Naruto in top shape during the process.
Sasuke is likely to be an important character. His power and his relationship with Naruto could play a big part in the story. He might help Naruto in the fight against the Juubi or have some other role related to the sealing process.
Well, considering Naruto's own experiences and the variety of seals in the Naruto universe, he could sell communication seals. These would allow ninjas to send messages over long distances. He might also sell seals that are related to elemental jutsu, perhaps ones that enhance or modify fire, water, wind, earth or lightning techniques.
Perhaps Naruto was in a situation where he had made a deal. For example, he promised to pay for the seals in exchange for some information about a missing nin or a secret technique. He values knowledge and power, and if paying for the seals was part of getting closer to achieving his goals, he would do it.
In a fanfic where Naruto studies seals in Kumo, he might learn unique sealing techniques not available in Konoha. He could meet new mentors or partners who are experts in seals in Kumo. This could lead to him developing new jutsu combinations using his newfound sealing knowledge and his existing skills like the Rasengan.