Most likely, Sam and Dean Winchester would be main characters. They are the central figures in the Supernatural series, and any fanfiction set in that universe would probably revolve around them. They are the ones who usually deal with the chaos and the supernatural threats.
In this type of fanfiction, the character development can be really fascinating. Since it's not bound by the show's exact plotline, the author can take a character like Castiel and develop him in unexpected ways. He could face moral dilemmas that are more extreme or form alliances that we wouldn't see in the original show. This allows for a deeper exploration of the characters' personalities and what makes them tick.
In such fanfiction, there could be chaos and intense battles among heroes and villains. The plot might thicken with unexpected alliances and power struggles.
In fanfiction, Dean as a Prince of Hell can open up many interesting storylines. One possibility is that his character becomes darker and more conflicted. He might struggle with his newfound powers and the evil that comes with being a Prince of Hell. His relationships with Sam and Castiel would also be severely tested as they try to figure out how to deal with this new Dean.
In many fanfictions, Dean as a Knight of Hell can bring a whole new dynamic to the Supernatural universe. His relationships with other characters, like Sam, would be severely tested. He could be seen as a threat by some and a potential ally by others who hope to control or redeem him.
Well, in such fanfiction, when the levee breaks, it might lead to a power struggle among different supernatural factions as they try to protect or take advantage of the situation. There could also be unexpected alliances formed or old rivalries reignited.
It usually has unique storylines and character developments that expand on the original supernatural theme. There might also be unexpected plot twists and deeper explorations of emotions.
Well, a big theme is identity. She is a princess of Hell, but she may question who she really is and what she wants to be. There's often a theme of isolation too. Being in such a powerful and often feared position, she may feel alone. And then there's the theme of family. In Hell, family can be a very complex thing. She might have to deal with her demonic family members and their expectations. For example, in some fanfictions, she has to prove herself worthy to her father, the King of Hell. There are also themes related to the exploration of Hell itself, like the different levels and what they represent in relation to her character.
In fanfiction, when Dean becomes a Prince of Hell, his personality could shift. He may start to see things from a more demonic perspective. His relationships with other characters would be redefined. For example, his bond with his brother Sam could be strained as Sam tries to save Dean from his new, dark self.