Perhaps they love fanfiction that focuses on character development. It doesn't matter what the original source is, whether it's a movie, a book, or a TV series. If the fanfiction delves deep into the characters' emotions, growth, and backstories, they would be interested. For instance, a fanfiction that explores the early life of a superhero in a comic - book universe could be something they love.
It's possible that Davis and Kari are fans of fanfiction related to fantasy novels, such as those in the Harry Potter or Lord of the Rings universes. Fanfiction in these areas can expand on the magic, the characters' relationships, and the overall world - building. For example, they might enjoy reading about alternate timelines or new adventures for the characters they already love.
They could love all kinds of fanfiction. Maybe they're into the fanfiction based on science - fiction shows, like Star Trek or Doctor Who. These shows have vast universes that can be explored in many different ways in fanfiction.