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.
One of the main themes is the battle between good and evil. Light, with the Death Note, represents a form of extreme justice that blurs the line between right and wrong. Another theme is the power of intelligence. Light and L constantly outwit each other using their brilliant minds. Also, it touches on the concept of morality. The use of the Death Note to kill people, even criminals, raises questions about who has the right to judge and take lives.
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.
I think Near is one of the most interesting characters. He has his own way of thinking and investigating, and he enters the fray at a crucial point in the story. He is different from L but still manages to be a formidable opponent. Then there's Matsuda, who is a bit of an underdog among the group. His more naive and emotional reactions compared to the other highly intelligent characters provide a different perspective in the complex world of 'Death Note Light Novel'.