Harry Potter himself could be a main character. He would be the one to sense the danger of the dark side dance and try to oppose it. Hermione would also be important as she could use her vast knowledge of magic to research about the origin and nature of the dance. Ron could be there to provide the comical relief but also be a loyal friend in the fight against the dark influence of the dance.
Draco Malfoy might be a main character in this fanfiction. He could be initially attracted to the dark side dance, seeing it as a way to gain more power and status. His inner conflict between his family's dark past and his own possible redemption could be explored through his involvement with the dance. And of course, Professor Dumbledore would be a crucial character. He would be the one guiding Harry and his friends from the background, providing wisdom and clues on how to deal with the threat of the dark side dance.
The plot could involve a new dark wizard who arrives at Hogwarts under the guise of a dance instructor. He teaches a dance that is actually a means of opening a portal to the dark side. The students who participate unknowingly start to have their minds filled with dark thoughts. Harry, with his natural resistance to the dark side, notices the change in his classmates and tries to stop the dance instructor before it's too late. He teams up with Hermione and Ron to figure out the true nature of this 'dark side dance' and close the portal.
Obviously, Harry Potter himself could be a main character. He could be the one who is initially reluctant to participate in a dance event but then discovers a new passion for it. Hermione could also be a main character as she might be the one researching the history and significance of different wizarding dances.
It could be Dumbledore. In a twisted take on the story, if Harry joins the dark side, Dumbledore would be the one trying to stop him. Dumbledore has always been the guiding force for the good side, and Harry's defection would be a huge blow. Dumbledore would then have to face the difficult decision of either trying to save Harry or stopping him by any means necessary, making him the antagonist from Harry's new 'dark' perspective.
It could be Dumbledore. If Harry turns to the Dark Side, Dumbledore, who has always been a guiding figure for Harry, would be the main one to oppose him. Dumbledore represents the light and everything Harry is now against. Dumbledore would try to stop Harry from causing chaos and destruction in the wizarding world.
Dumbledore could be seen as the main antagonist from Harry's new perspective on the dark side. Dumbledore represents all that is good in the wizarding world, and if Harry has turned to the dark, Dumbledore would be the one trying to stop him, to bring him back to the light. So in this context, Dumbledore becomes the opposition that Harry has to face.
Dumbledore could be a significant presence, even if he's no longer alive in the original timeline. His wisdom and knowledge could be passed on in some magical way, like through a magical diary or a portrait that has special powers. And then there could be some characters from the dark side who are trying to redeem themselves, such as a former Death Eater who sees the error of their ways and joins Harry's new movement in the fanfiction.
It's hard to say for sure without reading the fanfiction, but it could be Harry Potter himself. He might be at the center of the story, trying to figure out what's going on with this 'Dance of Dragons' in his world. Hermione could also be a main character, using her intelligence to research and understand the phenomenon. And perhaps some of the dragon keepers or those with a special connection to dragons in the Harry Potter world could be main characters too.
Obviously, Harry Potter himself would be the central character. Then, Voldemort would be a major figure as he is the main representative of the Dark Side. Bellatrix Lestrange could also play an important role as she is one of Voldemort's most loyal followers. She might be the one to initially tempt Harry or guide him into the ways of the Dark Side.
In a 'Harry Potter turns to the Dark Side' fanfiction, the plot might involve Harry being influenced by a powerful dark object. For example, a cursed artifact that makes him see the Dark Arts in a different light. He could start to believe that the ends justify the means and that the Dark Side has been misrepresented all along. As he delves deeper into the dark side, he begins to turn against his old friends, thinking they are too naive and weak to truly change the wizarding world.
Voldemort could still be a major antagonist, but in a different way. Since Harry is turning to the dark side, Voldemort might see him as a threat to his own power and try to eliminate him. Another could be Dumbledore. If Harry turns dark, Dumbledore would surely oppose him, trying to bring him back to the light. So Dumbledore could be an antagonist from Harry's new dark perspective.