A crossover with 'Fullmetal Alchemist' could also be fascinating. Maybe Edward Elric discovers the Death Note in his search for the Philosopher's Stone. He would be conflicted because his moral code as an alchemist goes against using such a tool for killing, yet the power it offers could potentially help him achieve his goals in a different way.
One possible plot could be that Light Yagami discovers the existence of demons from the Inuyasha world. He might see them as a new type of 'evil' to be eradicated. He could start using the Death Note on demons, which would draw the attention of Inuyasha and his group. Inuyasha would initially be confused about these strange deaths and then realize that a human is causing them. This would lead to a conflict as Inuyasha believes in slaying demons in a more 'honorable' way and not through this strange notebook's power.
How about a crossover with a post - apocalyptic world? The Angels of Death duo would find themselves in a world full of ruins and scarce resources. They would have to deal with new threats like mutated creatures and other desperate survivors. Their relationship could be further tested in this harsh new environment.
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 plot idea could be Ryuk trying to save Light from the consequences of his actions. Since Ryuk is an otherworldly being, he might have some knowledge or power that could potentially help Light avoid getting caught. For example, he could use his Shinigami powers in a way that no one expects to mislead the investigators on Light's trail.
Well, in scp death note crossover fanfiction, a cool concept is the clash of different power systems. The SCP entities have their own unique and often bizarre abilities, and when combined with the Death Note's ability to kill just by writing a name, it creates a really chaotic and interesting scenario. Maybe some SCPs would try to find a way to nullify the Death Note's power or use it to their own advantage in their never - ending pursuit of chaos or control.
Well, in 'kaneki crossover death note fanfiction', a possible concept is the way their different worlds collide. Kaneki comes from a world full of ghouls and the CCG, and Light is in a world where he's using the Death Note to change society. If they were to meet, it could lead to some really intense power struggles. Also, the idea of how their respective allies and enemies would react to the crossover is quite fascinating. Kaneki's friends from Anteiku might be completely bewildered by the Death Note, and Light's accomplices might see Kaneki as a new threat or a potential tool.
One idea could be that it's an alternate universe where Light's actions in the Death Note world have led to some strange consequence that results in a pregnancy - like situation for him, but it's more of a metaphor for a new 'life' or change coming into his world. For example, he could be 'pregnant' with a new idea or plan that will change the power dynamics in the Kira case.
One idea could be a story where Light and Misa try to navigate their relationship outside of the Kira case. Maybe they go on a trip together and have to deal with normal couple problems while still being on guard against possible pursuers who might suspect their true identities.
Well, a possible fanfiction concept could be that Sakura enters the world of Death Note. She might meet Light Yagami and be appalled by his actions. So, she decides to use her medical knowledge (since she's a medic in Naruto) to find a way to counter the effects of the Death Note, or at least expose Light without using the unethical power of the Death Note herself. It could be a story of her moral struggle in a world with different rules of life and death.