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.
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.
Ryuk would probably just be amused. He is a Shinigami who has seen a lot in his existence. Fanfiction is just another strange human creation to him. He might chuckle at some of the more outlandish ideas in the fanfictions, like a character having superpowers beyond the Death Note.
Misa would likely be excited. She is a big fan of Light and the whole 'Kira' concept. In fanfiction, if there are scenarios where she has more power or a closer relationship with Light, she would be all for it. However, she might also get jealous if there are other female characters getting too close to Light in the fanfictions.
Oh, for sure! Death Note is considered a masterpiece in the manga world. Its unique concept and the way it unfolds make it a must-read. The characters' development and the moral dilemmas presented make it stand out from the crowd.