Well, obviously Sirius Black could be a main character. He was always a bit of a rebel and would fit right in with the Hellfire Club's atmosphere. He could be the one leading the more adventurous and rule - breaking activities within the club.
Well, aside from the Marauders like Sirius, Remus, and James, Severus Snape can be a very popular character in Hellfire Club fanfiction. His relationship with the other characters, especially his rivalry with James Potter, can add a lot of drama. Also, Narcissa Malfoy could be an interesting character to include. Her family's influence and her own skills can create some unique storylines within the context of the Hellfire Club. And don't forget about Albus Dumbledore. His presence, whether as an ally or a figure to be reckoned with, can make for some fascinating plot developments.
I don't know for sure as I haven't read it. But it could be the founders or leaders of the Hellfire Club.
I'm not entirely sure without reading it, but likely there are powerful and influential figures within the club as main characters.
Since it's a fanfiction, it could still have Harry Potter as a main character. Maybe Hermione and Ron are also involved. There could also be some original characters created specifically for this fanfiction that are part of the Hellfire group.
One idea could be that a new student joins the Hellfire Club and has to prove their worth in a series of magical challenges designed by the club's members. These challenges could involve things like finding a hidden magical artifact within Hogwarts that is protected by various spells.
Draco Malfoy might be an interesting main character in this fanfiction. He could be in the Host Club as a way to redeem himself or to show a different side of his personality. Luna Lovegood would also be a great choice. Her unique perspective and dreamy nature could make the club more interesting and she could offer services like sharing her strange but fascinating knowledge about magical creatures.
One interesting plotline could be the exploration of new magical abilities within the Hellfire Club. Maybe some members discover rare spells or powers that are not part of the regular Hogwarts curriculum. Another could be the relationships between different characters in the club, like how they interact and form alliances or rivalries. Also, a plotline could involve the club getting involved in a mystery that even Dumbledore has trouble solving.
In some 'hellfire dragon slayer fanfic', there could be a powerful wizard as a main character. This wizard might not be a direct slayer but has the knowledge and power to help the slayer in their quest. And of course, the hellfire dragons can be considered main characters in a way, as their actions and nature drive much of the story's conflict.
You could have a new generation of witches and wizards as the main characters. For example, the children of the original trio. They might be visiting the manor for the first time and discover something that their parents had missed. There could also be a misfit wizard who finds a sense of belonging at the manor, and his story could be the focus.
In a 'harry potter inquisitor fanfic', the main characters could include the Inquisitor, who is the central figure driving the investigations. Then there could be a new character, an exchange student from a far - off magical school. This student has a different approach to magic and catches the Inquisitor's eye. Additionally, Professors Snape and McGonagall could be main characters. Snape might be secretly working against the Inquisitor if he thinks their actions are unjust, while McGonagall could be trying to keep the peace between the Inquisitor and the students. The interactions between these characters would create a rich and engaging story.