Voldemort is, of course, a major figure. In WBWL fanfictions, his interactions with Harry are complex. He might initially see Harry as a threat even in this 'wrong' scenario, but then later try to turn him to his side. Other characters like Hermione and Ron may be portrayed as enemies or at least estranged from Harry in these darker interpretations.
Bellatrix Lestrange can be a significant character in these fanfictions. Since Harry is on a darker path, she might see him as a potential recruit or someone she can manipulate. Sirius Black may also be present, but his role could be one of trying to save Harry from his dark descent, which adds a lot of conflict to the story.
I think Sirius Black can be quite interesting in these fics. In a world without Harry, Sirius might have a different path. If he becomes an assassin, his loyalty and his wild nature would be put to a different use. He could be fighting against the corrupt Ministry of Magic or other dark forces in a more direct and violent way than in the original story.
In some WBWL fanfictions, Harry is ostracized by the wizarding world because they believe he's not the 'true' savior. This makes him turn dark. He might form an alliance with Voldemort's followers out of spite. The plots often explore how his personality changes. He could become more ruthless and calculating, using his natural magical talent in ways that are more self - serving rather than for the greater good.
One popular 'Harry Potter Dark WBWL Fanfiction' is 'The Dark Lord's Heir'. It delves into a world where the 'Boy-Who-Lived' takes a much darker path. Another one could be 'Darkness Reborn' which has an interesting take on how the alternate Harry Potter character uses his powers in a more sinister way. There's also 'Shadow of the Prophecy' that plays with the idea of a dark destiny for the supposed hero.
Voldemort is quite common. Since he's the main villain in the original series, fanfic writers often use him to create darker storylines. Bellatrix Lestrange is also popular. Her insane and violent nature gives a lot of room for dark interpretations.
Voldemort often appears as a sort of 'mentor' figure in some dark path fanfictions. His ideology and power attract characters to the dark side.
Obviously, Harry Potter is one of the main characters. And the Malfoy family members, like Lucius, Narcissa, and Draco Malfoy are also main characters as Harry is raised by them.
Another theme is gender roles. It shows how a female in the position of the 'Boy Who Lived' challenges the traditional gender roles in the wizarding society. This can be seen in how she interacts with male and female characters, and how she's treated by others. In some fanfics, she has to fight against the preconceived notions of what a female can and can't do in the magical world.
Power is also a common theme. In these fanfictions, the WBWL character may have different powers or a different way of using magic compared to Harry. They might have to learn to control and use their power in a world that has certain expectations based on what Harry Potter could do. And family plays a role too. How the character's family, whether it's their birth family or the family they find along the way, affects their journey in this alternate scenario is often explored.
One common theme is identity. Since the main character is a female Harry in a world where things have changed due to Voldemort marking Neville instead, she has to figure out who she is in this new context. Another theme is friendship. How her relationships with characters like Ron and Hermione might be different. For example, in some fics, the female Harry might have a closer bond with Hermione as they can relate more as girls.
In Harry Potter Dark Arts fanfiction, Voldemort is often a very popular character. His power and his connection to the Dark Arts make him a central figure in many stories. Another popular character is Severus Snape. His complex relationship with the Dark Arts and his dual nature of being both a potential ally of the Dark side and a hero on the side of good add a lot of depth to fanfictions. Bellatrix Lestrange is also popular as she is a fanatical follower of the Dark Arts.