One possible plot development could be that the village is in great danger from an enemy too powerful to defeat directly. Naruto, seeing no other way, seals himself away to act as a last - line - of - defense deterrent. If the enemy tries to completely destroy the village, Naruto's sealed power could be unleashed to counter them.
It could be a new and extremely powerful ninja from a far - off land who has a vendetta against Konoha. His power might be so great that Naruto decides the only way to stop him in the long run is to seal himself away as a sort of trap for when the antagonist gets too close to achieving his goals.
In some versions of fanfiction, Naruto might use a new and powerful sealing jutsu that he has either created or learned specifically for this purpose. It could be a jutsu that combines his Uzumaki heritage's sealing knowledge with his own unique chakra control.
One consequence could be that Naruto has a constant struggle to keep Kaguya's power in check. He might experience internal battles within his mind as Kaguya tries to break free.
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.