I'm not sure specifically as there could be many different plots in a fanfic about them. It might involve their relationship developing, perhaps going through some sort of adventure or emotional journey together.
We don't have enough information to know exactly who Karen is. She could be an ordinary person, a student, a teacher, or just someone who likes to read stories.
Senjoughara and Araragi are the main characters in this fanfic. Senjoughara might bring her own charm and strong - willed personality to the story, while Araragi with his own set of traits, and together they form the core of the narrative.
One common theme is self - discovery. Both characters are on a journey of understanding themselves better, and their relationship often serves as a catalyst for that. For example, in many fanfictions, Araragi realizes new aspects of his personality through his interactions with Senjougahara.
One common theme is the exploration of their complex relationship. Given Shinobu's vampiric nature and Araragi's human - turned - vampire nature in a way, their relationship is full of power dynamics, loyalty, and a strange form of mutual dependence.
Given only 'dateline karen read story', Karen's identity remains a mystery. She might be a staff member at Dateline who was reading a story for work purposes. Or she could be a person who was interviewed by Dateline and was reading a story during the interview, or she could be an ordinary person whose story - reading experience is being highlighted in relation to Dateline.
One interesting plot could be a story where Senjougahara and Araragi go on a mysterious adventure together. They might encounter some supernatural beings and have to rely on their bond to overcome the challenges. This could deepen their relationship in unexpected ways within the context of the fanfiction.
A common theme is the power dynamic between them. Given Shinobu's powerful vampire nature and Araragi's somewhat normal yet complex character, their relationship often plays with who has more influence at different times.