An ancient spirit that was trapped by the original creators of the Reaper Death Seal and has now been released. This spirit wants revenge on the shinobi world and uses its knowledge of the seal to cause chaos.
A rogue shinobi who has found a way to corrupt the Reaper Death Seal and use it for his own evil purposes. He could be stealing the souls of innocent people instead of using it as a last - resort jutsu like it was intended.
Perhaps an unknown hermit living in some far - off place. In many fanfics, there are mysterious characters that pop up with unique knowledge. This hermit could have mastered the Reaper Death Seal long ago and sees Naruto's potential, so decides to teach him.
In this fanfic, Naruto might face some really tough challenges while learning the Reaper Death Seal. He could be in a situation where he has to sacrifice a lot to master it, like maybe losing some of his relationships temporarily. Also, he might have some sort of spiritual journey to fully understand the power of the seal.
It could be an old enemy of the village who has been waiting for the seal to break. Maybe someone who was imprisoned by the same seal long ago and now wants revenge on the village for sealing them away.
Danzo could be a great antagonist in this fanfic. He was always power - hungry and had his own secretive ways. He might see the five pronged seal as a way to control Naruto or to use his power for his own dark purposes. He could try to manipulate Naruto or those around him to keep the seal in place or even make it stronger for his own gain.
One possible storyline could be that Naruto is faced with an overwhelmingly powerful enemy that he realizes can't be defeated any other way. So, in a selfless act, he decides to use the Reaper Death Seal. This would be a very tragic end for Naruto as he has so many friends and a family to live for. His death would likely send shockwaves through the Hidden Leaf Village and among his friends like Sasuke, Sakura, and Kakashi. They would be devastated, and it might also lead to a power vacuum in the village, causing all sorts of political and social unrest.
Another plot idea is to have a character who is not Naruto, perhaps a new OC (original character), inherit the knowledge of the Reaper Death Seal in an unexpected way. This character could then face the moral dilemma of using such a powerful and dangerous jutsu.
It could be an ancient dark ninja who wants to use Naruto's new 'avatar of death' powers for his own evil purposes. This ninja might have been researching death - related jutsus for years and sees Naruto as the key to his ultimate power.
One popular 'naruto reaper death seal fanfiction' is 'The Seal's Legacy'. It delves into the after - effects of the death seal on Naruto's world. Another interesting one could be 'Reaper's Redemption' which focuses on how the reaper aspect in the death seal might have a different agenda than initially thought.
Power and sacrifice are common themes. The death seal is a very powerful jutsu that often requires sacrifice, so many fanfictions explore this aspect. For example, a character might have to sacrifice their own life or something precious to use the seal effectively.
It could be a rogue Kumo ninja who wants to steal Naruto's seal - mastering knowledge for his own evil purposes. Maybe he wants to use the seals to take over Kumo or even the whole ninja world.