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.
A plot could revolve around a magical or scientific experiment gone wrong in the Death Note universe that somehow makes Light have a condition similar to pregnancy. This would then set off a chain of events where he has to deal with the physical and emotional implications. It could be used to show a more vulnerable side of Light, his growth as a character, and how he interacts with the people around him in this strange new situation, all while keeping the story appropriate for all readers.
One idea could be the two of them preparing for the baby. Historia might be learning about different parenting styles, and Eren could be building a nursery. It could show their growth as individuals and as a couple.
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.
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.
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.
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.
A possible plotline could be a character finding out they are pregnant in the middle of the 'Resident Evil' chaos. For example, a female survivor in a post - apocalyptic setting might be pregnant and her journey could be about protecting her unborn child from the infected and the harsh environment. She would have to find food, shelter, and safety all while being pregnant, which adds a new layer of drama and survival challenges.
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.
Perhaps a fanfic where L is at the bottom of his game, almost defeated by Light. But then he finds an unexpected ally in a character who was previously thought to be insignificant. This ally has a unique ability that helps L turn the tables in their battle of wits over the Death Note.
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.