One possible plot could be Katherine coming back from hell with new powers. Maybe she seeks revenge on those who wronged her before. She could target Elena specifically, creating a lot of chaos in Mystic Falls.
Perhaps she forms an unlikely alliance with other damned souls in hell. They might work together to try and find a way to escape or at least improve their situation in hell. It could involve some sort of power struggle among different groups of souls in the hellish realm.
Obviously, Katherine would be one of the main characters as the story is centered around her time in hell and what happens after. Damon and Stefan would also be important as they have a long history with Katherine. Their reactions to her return would be crucial to the plot.
It's possible that she'll form new relationships in hell that will change her. For instance, if she meets a soul who was a victim of her actions on Earth, she could be forced to confront the consequences of her actions directly. This could lead to a transformation in her personality, either towards redemption or further into villainy depending on how she reacts to these new relationships.
Perhaps it could explore her past actions in more detail while she's in hell, like how her wrongdoings on earth are now being punished or how she reflects on her relationships, especially with Stefan and Damon. It could also show her facing her inner demons, both literally and figuratively, as she tries to find a way out of the hellish realm.
One possible plotline could be about Enzo protecting his daughter from the various threats in the TVD universe. Maybe she has a special power that makes her a target, and Enzo uses his vampire skills to keep her safe.
Perhaps Katherine kidnaps Elena out of jealousy. Elena has the love and attention of many people that Katherine once had or desired. So, Katherine might take Elena to a remote location, taunt her about all the things she doesn't have, and try to make her feel inferior. In this fanfiction, Elena could then try to outwit Katherine using her own wits and the help of her friends she manages to contact secretly.
Well, a plot could revolve around the father of the baby. He might be in a difficult situation, like being trapped in some sort of supernatural bargain that endangers the pregnancy. Or perhaps the pregnant character forms an unexpected alliance with a former foe because they both have an interest in the well - being of the unborn child. It could add a lot of drama and new dynamics to the fanfiction.
Obviously, Katherine herself. Also, there could be appearances from demons or other hellish creatures that are part of the hell setting. Given her history, Stefan or Damon might make appearances in her memories or even in some sort of spectral form if the fanfiction has elements of the afterlife being connected to the living world.
Well, it could be about the family dynamics within the 'TVD' (The Vampire Diaries) universe. Maybe it focuses on a particular family's secrets, like the Salvatores or the Gilberts. It might explore how family relationships are affected by the supernatural elements in the show.
One possible plot could be that Harry is sent to Hell due to a powerful dark magic spell gone wrong. In Hell, he has to face his worst fears and demons, both literal and metaphorical. He might encounter lost souls who tell him secrets about the magical world. Eventually, he manages to find a way back, using his wits and some unexpected magical abilities he discovers during his ordeal.