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 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.
She would likely become more self - reliant. Being in prison, she can't rely on her usual support system. So she has to figure out how to survive on her own, which would make her tougher. For example, she might learn how to endure harsher conditions without breaking.
Shippo might become more self - reliant. When he travels through time, he may be separated from his friends. This would force him to rely on his own skills and intelligence to survive. For example, if he travels to a time where he doesn't know anyone, he has to figure out how to get food and shelter on his own.
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.
Ichigo's character could develop a greater sense of patience. In different time periods, he might have to wait for the right moment to act or to understand the events around him. For example, if he travels to a more primitive time in the Soul Society, things may not happen as quickly as he's used to in his own time. This would force him to be more patient. He could also become more empathetic as he experiences the hardships of different eras and the people in them.
Enzo would become more responsible. As a vampire, he was often carefree, but having a daughter would make him think about someone else's well - being first. He'd be more cautious in his actions.
If Tsunade time travels, she might become more cautious. Seeing the past mistakes of the village, she'd be more careful in making decisions in the present. For example, if she saw how a certain policy led to problems in the past, she'd oppose similar ones in her own time.
She might become more empathetic. Meeting people from the past, especially those who were suffering or in difficult situations, could make Serena more understanding. She could see the world from a different perspective and this would change how she interacts with others in her own time. For example, if she sees how people in the past fought for equality, she may be more involved in similar causes in her present.