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.
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.
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.
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.
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.
I really like 'Klaus and the Unlikely Companion'. In this fanfiction, the OC is a witch who crosses paths with Klaus. What makes it stand out is the way it portrays Klaus's vulnerability around the OC. It shows a different side of him that we don't often see in the show. The plot twists are also quite engaging.
A 'a family affair tvd fanfic' could be about different family - related storylines. It might be about a family that has just moved to Mystic Falls and how they get involved with the existing families in the show. Or perhaps it's about a long - lost family member of one of the main characters showing up and causing chaos. It could also explore the family trees of the vampires and werewolves in the show, and how their family legacies impact their actions and interactions.
I'm not sure specifically as it could be various things. 'TVD' might refer to 'The Vampire Diaries'. It could be a fan - created story about a 'burned angel' within the context of The Vampire Diaries universe. Maybe it's about an angel - like character who has been through some sort of burning or traumatic event and how that character interacts with the other elements and characters from the show.
One great 'TVD Caroline pregnant' fanfiction is 'Caroline's New Chapter'. It beautifully explores Caroline's journey through pregnancy while dealing with the supernatural world around her. The author does a wonderful job of portraying her strength and the support she gets from her friends.