One potential 'Dark Lord' could be Lelouch. Since he already has the power of Geass and is a complex character, it's easy to imagine a scenario where he succumbs to the darker side of his power. His intelligence and strategic mind could be used for more malevolent purposes.
It could be that the 'Rise of the Dark Lord' is a consequence of some event that went differently in the fanfiction compared to the original series. For instance, if Lelouch made different choices regarding the use of his Geass. Maybe he became corrupted by the power and started to rule with an iron fist, thus becoming the 'Dark Lord'. This would change the relationships he had with his friends and allies, like Suzaku. And the story would be about the chaos and destruction that follows as he ascends to this dark position, and the efforts of those who oppose him to bring him down.
It could be a powerful, ancient Titan who was banished long ago and has now returned for revenge. Maybe Kronos has an even more evil and powerful form that he takes on, becoming the Dark Lord. He wants to destroy the new order of the gods and demigods that has been established since his last defeat.
The dark lord himself is obviously a main character. There may also be a young hero or heroine who stands against him.
Hermione could be one of the characters in the harem. Her intelligence would be an asset to Harry, even as a Dark Lord. She might be torn between her loyalty to the original Harry she knew and her feelings for this new, darker version. Ginny could also be part of it. Given her connection to Harry in the original story, her presence in the harem could add an interesting dynamic, especially if she tries to change Harry back to his old self.
Sabrina could also find an unexpected ally in a magical creature. Maybe a talking cat or a friendly spirit that has knowledge about the Dark Lord's weaknesses. This creature could guide Sabrina on her journey to either defeat or at least resist the Dark Lord. It could be that this creature has a long - standing grudge against the Dark Lord and sees Sabrina as a means to finally get revenge.
In the 'rise of the dark lord novel', the main character is likely to be the dark lord. However, there may also be characters like his minions or followers who play important roles in his rise. On the other side, there could be a wizard or a knight who tries to stop him, and they would also be main characters in the sense that they drive the plot in opposition to the dark lord.
Maybe a powerful wizard who is jealous of the power of both the Shield Hero and the Dark Lord. He wants to eliminate them and take their powers for himself. He could be creating false rumors about the Dark Lord to keep the Shield Hero against him while he plots his own takeover.
I'm not sure specifically as I haven't read it in detail. But generally, it might be about the ascension or the growing power of a dark lord, perhaps his origin, his evil plans, and the challenges that the protagonists face against him.
It could also be an existing character from the Harry Potter series like Hagrid. Given his love for all creatures, including some that are considered dangerous, he could be at the center of a story about the 'Lord of Dark Creatures'. He might be trying to protect these creatures from those who would use them for evil purposes or trying to rehabilitate them.
Well, Nico di Angelo has a good chance of being a main character in this kind of fanfic. His powers over the dead and his somewhat dark and brooding nature could make him a key figure in the fight against the dark lord. Thalia Grace, with her leadership skills and powerful abilities as a daughter of Zeus, would also be a strong candidate. And then there's Luke Castellan. If the fanfic is set in a different timeline or has some sort of alternate reality, Luke could be brought back in a new role, perhaps as a redeemed character who helps in the battle against the dark lord after having once been on the wrong side.