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.
Bonnie could be a main character too. Since she has strong magical abilities, she might be the one to sense Katherine's return first or even try to stop her if she thinks Katherine is up to no good. Also, Caroline might get involved. Given her strong personality, she could be a key player in dealing with Katherine's return, whether it's trying to protect the town or trying to rehabilitate Katherine if she shows signs of change.
In addition to the regulars, there could be some new characters introduced. Maybe a demon from hell that Katherine made a deal with follows her to Mystic Falls. This demon could have its own motives and powers, and it would add a new layer of danger and mystery to the fanfiction. Or perhaps an old acquaintance of Katherine's from her past in hell shows up, someone who knows all of her secrets and could either help or hinder her in her new life above ground.
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.
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.
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.
There could also be some minor characters from other families that become important in this fanfiction. For instance, a distant relative of the Lockwood family who shows up and causes a stir in the family affairs. They could bring new secrets and power dynamics to the story, changing the way the existing families interact with each other.
It could be someone who misuses the power of the Hell Girl. Maybe a rogue ninja who discovers the way to summon the Hell Girl and uses her to get rid of his enemies in the ninja world. He doesn't care about the consequences or the morality of it, and Naruto has to stop him from causing more chaos.
A vampire and a werewolf who are both outcasts in their respective supernatural communities. They meet in a place that is like a hell on earth, a desolate and cursed land. They have to work together to survive against hunters and other threats, making them a 'match made in hell' in the sense that their species are natural enemies.
The main characters are clearly Elijah and Katherine. They are the central focus of this fanfiction, especially since the title mentions them directly and the pregnancy is related to them.
During the pregnancy in the TVD After 8x16 fanfiction, relationships are bound to shift. The mother - to - be's relationship with her doctor, if there is a supernatural - themed doctor in the story, could be one of trust and dependence. Her relationship with other pregnant women in the supernatural community, if there are any, could be a source of support and shared experiences. Also, the relationship between the unborn child's potential godparents and the mother could be redefined as they take on more responsibilities and make promises for the future of the child.