Perhaps there are dark wizards who don't want Harry to gain more power through this lordship. They might fear that he'll use it to oppose them more effectively. So they would plot to stop him from fully taking on his new role.
There could also be some within the Ministry of Magic who oppose Harry's new status. They might think he's too young or inexperienced to handle such a position. These bureaucrats could create all sorts of obstacles for Harry as he tries to understand and embrace his unexpected lordship.
Well, it could be about Harry suddenly finding out he has some sort of lordship that was previously unknown. Maybe it's related to an old wizarding family. He might have to deal with new responsibilities, like managing estates or dealing with family secrets.
Obviously, Harry Potter himself is the central character. Since it's about his unexpected lordship, he is the one going through all the changes and challenges. Hermione Granger is also often involved. She helps Harry understand the implications of his new status, whether it's researching about the history of the lordship or using her intelligence to solve problems related to it. Ron Weasley is there too, providing his usual loyal support and comic relief.
Well, in some 'Harry Potter Unexpected Lordship' fanfictions, Harry might discover a hidden lineage that makes him an unexpected lord. For example, he could be related to an ancient wizarding family that was thought to be extinct. This could lead to him having new powers, allies, and enemies. Another plot could be that he inherits a lordship through some sort of magical will or prophecy, and he has to navigate the complex politics of the wizarding world as a result.
It could be the Ministry of Magic. They have often been shown to be corrupt and self - serving in the original series. If they try to manipulate Harry again, he might see them as the main antagonists and target them in his revenge.
Obviously, Voldemort could be one of the main antagonists. He might be the one who lures Harry into the League of Doom in the first place, with false promises. Another could be Bellatrix Lestrange. She is extremely loyal to Voldemort and would be very suspicious of Harry's presence in the league and try to expose or kill him.
Some pure - blood supremacist wizards could be antagonists. They don't like the idea of Harry creating a haven for all kinds of wizards on the island. They might try to sabotage the island's creation or attack it once it's made, fearing that it will disrupt the 'purity' of the wizarding bloodlines.
Obviously, Harry Potter is one of the main characters as the title suggests. There might also be characters like Hermione and Ron, his best friends, who would likely be involved in his journey with this new lordship. And perhaps some other characters from the original Harry Potter series like Dumbledore (if it's set in that time frame) or other members of the wizarding community who have an interest in this new development in Harry's life.
Perhaps some other characters who misunderstand Lily's 'insanity' could turn into antagonists. For example, if the Hogwarts faculty or some of her peers think she's a danger because of her erratic behavior, they might try to isolate or stop her, creating a sort of antagonistic relationship.
Harry Potter is often a main character. Since he's the central figure in the original series, it's natural for him to be involved in fanfiction about lordships. Hermione Granger might also be there as she is Harry's closest friend and often gets involved in important matters. Ron Weasley could be another, as he's part of the trio.
Voldemort or his followers might also be antagonists. Even if Voldemort is defeated in the original story, in this fanfiction, his remaining Death Eaters could see Harry's new power as something they can either use or must destroy. Or perhaps a new dark wizard inspired by Voldemort's ideology rises up against Harry.