Some Death Note fanfiction focuses on the idea of what if there were multiple Death Notes in the world at the same time. This would lead to chaos as different users with different motives fight for power and try to outwit each other, creating a complex web of plots and counter - plots.
A fanfiction might focus on the Shinigami. For example, Ryuk could be the main character in a story where he decides to interfere more in the human world. He could start to manipulate events in a way that benefits him or just for his own amusement. This would give a different perspective on the Death Note universe, showing the Shinigami's view of the humans and their actions.
One interesting 'Death Note Reaction Fanfiction' could be about the characters' reactions when they first discover the Death Note. For example, Light's initial shock and then his quick realization of the power it holds. Another might focus on how L reacts to the existence of the Death Note and Light's actions. There could also be fanfictions that explore the reactions of secondary characters like Misa when she gets involved with the Death Note's power.
One interesting Death Note Genius OC Fanfiction could be about an OC who has the power to reverse the effects of the Death Note. This OC might use this power to save those who were wrongly targeted. Another could be an OC detective who is as brilliant as L and is able to outwit Kira. There are also fanfictions where an OC is a Shinigami who befriends Light and tries to change his fate.
One interesting fanfiction could be a story where the characters from Harry Potter somehow get their hands on a Death Note. Maybe Harry discovers the Death Note in the Forbidden Forest. He would be faced with the moral dilemma of using it against Voldemort, which is a completely different way of fighting compared to his usual magic. Another could be Hermione studying the rules of the Death Note and trying to find a loophole to defeat Death Eaters without actually killing them directly.
Perhaps a story where the Akatsuki members from Naruto are intrigued by the power of the Death Note. They could try to obtain it for their own purposes, like achieving world domination in a different way. It would be interesting to see how characters like Itachi or Kisame would react to such a strange and powerful object that is completely different from their ninja techniques.
An interesting OC fanfiction could be about a Death Note owner who doesn't want to use it for evil but for good in a very specific way, like only targeting truly heinous criminals who keep escaping justice. They could have moral dilemmas throughout the story.
One interesting crossover could be Batman getting his hands on the Death Note. He would likely be very conflicted about using it. On one hand, he could potentially rid Gotham of its worst criminals without having to physically fight them all. But his strong moral code would probably prevent him from using it, as it goes against his belief in justice through the legal system. Another aspect could be L from Death Note trying to figure out Batman's identity, using his detective skills which are somewhat similar to Batman's own.
There might be a fanfiction where L discovers Light's identity as Kira much earlier, but instead of immediately trying to expose him, he decides to study Light's motives and actions more closely. This could involve some intense psychological battles between them as L tries to figure out what makes Light tick and Light tries to outwit L in a different way.
One interesting story could be about how different characters interact with the Death Note in unique ways. For example, some might use it for what they consider 'justice' while others might be more self - serving. The short stories often explore the moral gray areas that the main Death Note series also delved into.
One interesting concept could be Naruto getting his hands on the Death Note. Imagine how his ninja values would conflict with the power of the Death Note. He might be tempted to use it for what he believes is justice in the ninja world, but his sense of right and wrong within his own moral code would be put to the test.