Well, in some fanfictions, memory seals can be hierarchical. There could be a main seal that controls access to a set of smaller, more specific memory seals. For example, a character might have a top - level seal that guards their entire past, and within that, there are smaller seals on specific events or people. To access a particular memory, one would need to break through the relevant sub - seal first. This makes the process of uncovering memories more complex and interesting.
Fanfiction about 'Dark Naruto, Kyuubi, Memory Seals' can take many forms. One possibility is that the memory seals are tampered with by an outside force. This could be an enemy who wants to unleash the power of the Kyuubi and corrupt Naruto. The 'Dark Naruto' that emerges might have different abilities compared to the regular Naruto. He could be more aggressive and less hesitant to use the Kyuubi's power without the moral restraints that the original Naruto has due to the disrupted memory seals.
In Naruto memory seals fanfiction, the idea of using memory seals for emotional protection is also cool. A character could have a seal placed on memories of a traumatic event. This can then be explored when the seal starts to break or when another character tries to help the sealed character face those memories. It adds a lot of depth to the character's psychological state.
In fanfiction, the Kyuubi's power is often a catalyst for the creation of Dark Naruto. Memory seals are used to control the Kyuubi's power within Naruto. If these seals are broken or weakened, the Kyuubi's influence can turn Naruto 'dark'. For example, the Kyuubi's chakra might seep into Naruto's mind and distort his thoughts, and the broken memory seals can't prevent this transformation.
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.
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.
The seals could also be a plot device for a darker storyline. Maybe they were placed by an unknown enemy to control Naruto or use him as a weapon. In some fanfictions, the seals start to affect Naruto's mental state, causing him to have inner struggles between his normal self and the influence of the seals.
The idea of Naruto sealing the Juubi is quite common in Naruto fanfictions. If Kurotsuchi is involved, it could be that the author wants to explore a different kind of relationship. For example, Kurotsuchi could be from a different village and her interaction with Naruto during this momentous event could lead to a new alliance or at least a better understanding between their respective villages. Also, the sealing of the Juubi by Naruto is no easy feat. It would require a great deal of chakra control and knowledge of sealing jutsus. Kurotsuchi, with her own skills in earth - based techniques, might be able to contribute in unexpected ways, like creating a stable environment for the sealing to take place.