One idea could be to create a Shinigami OC who is a bit of an outcast among the Shinigami. Maybe this OC doesn't like the idea of just writing names in the Death Note and wants to find a more 'humane' way to deal with humans, which could lead to all sorts of interesting conflicts with other Shinigami. For example, they might try to influence humans in a positive way before their time is up instead of just causing their death.
It focuses on the story of L's early days and how he solves cases. In this novel, we see L's unique detective methods and his battle of wits against various criminals.
First, consider their backstory. A Shinigami's past can greatly influence their present behavior. For example, if they were once a human who had a tragic end, it could shape their view on death and how they interact with living beings. Second, think about their personality traits. Are they cold and calculating like most Shinigami, or do they have a softer side? Maybe they're curious about humans in a way that other Shinigami aren't. Third, their relationships with other characters matter. Do they have a rival among the Shinigami? Or perhaps a mentor? These relationships can add depth to the character.
One possible fanfiction could be where Light is in the hospital due to an unexpected illness. Maybe he uses this time to plot his next moves in the Death Note world without arousing too much suspicion. He could be pretending to be weaker than he actually is while observing the people around him, looking for potential Kira supporters or enemies among the hospital staff and patients.
In this fanfiction concept, Naruto becoming the Shinigami could bring a whole new dynamic to the Naruto universe. He could use Shinigami - like powers for things like judgment and dealing with the souls of his enemies. It might also change his relationships with characters like Sasuke, as he now has a different kind of power and responsibility.