L's death in the manga occurs as a result of Light's elaborate schemes. Light finds a way to trick L and eliminate him, marking a significant turning point in the story.
One interesting Death Note L OC fanfiction could be where the original character has a unique ability to read L's thoughts. This adds a whole new layer of mystery and interaction between the two. Another could be an OC who is a detective from another country and teams up with L to solve Kira cases. There are also fanfictions where an OC is a childhood friend of L and has some secrets related to the Death Note.
The pairing of L and Misa is interesting because they are such different characters in the original 'Death Note' series. L is highly intelligent and logical, while Misa is more emotional and impulsive. In fanfics, seeing how these two contrasting personalities interact and potentially develop feelings for each other is fascinating.
I really liked 'The Unlikely Bond'. It's a fanfic that starts with L being intrigued by Misa in a way he didn't expect. The writing is quite engaging as it shows how their interactions gradually change from suspicion (as in the original series) to a sort of understanding and then something closer to affection. It's a slow - burn story that is really enjoyable to read.
One possible plotline could be the shock and confusion among the other characters. Since L's pregnancy is so unexpected, it could disrupt the power dynamics in the story. For example, Light might try to use this situation to his advantage in their battle of wits.