All of his so - called friends could be antagonists if they turn on Harry for some reason. Maybe they are jealous of his power or fame, and if they plot against him, Harry would surely see them as enemies worthy of revenge.
The entire Hogwarts faculty might be seen as antagonists if they side with the wrong people or try to stop Harry from getting justice. For example, if they enforce unfair rules against Harry, he could consider them as part of the problem and seek revenge.
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.
Hermione would be a likely supporter. She is always loyal to Harry and if he presented her with evidence of Dumbledore's wrongdoings, she would probably side with him. She has the intelligence to help Harry strategize and find ways to expose Dumbledore. For example, she could use her research skills to dig deeper into Dumbledore's past.
Perhaps Harry uncovers a conspiracy where all his friends, the teachers at Hogwarts, and even the Ministry of Magic were in on a plot to control him. He then decides to take revenge in a very calculated way. He starts by sabotaging the Ministry's operations secretly. He might use his invisibility cloak to move around undetected and cast spells that disrupt their meetings or plans. For his friends, he could play mind games, making them feel guilty about their actions without revealing his true intentions at first.
Dumbledore, even if not in person, could have left behind some clues or resources that help Harry. Also, the Order of the Phoenix members who are still around and believe in Harry's cause. They have their own skills and connections that could prove useful. For example, Moody could teach Harry some advanced defensive magic techniques to use against the Ministry's security measures.
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.
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.
Perhaps it could be a group of bullies from Issei's school days. They used to pick on him and now he's coming back to make them pay.
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.
Voldemort could still be a main antagonist. Even if Fem Harry has a new inheritance, Voldemort would see her as an even greater threat. He might try to steal her inheritance or stop her from using it to defeat him. His Death Eaters would also be involved, trying to carry out his evil plans against her.
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.
Maybe Dumbledore, if he were still alive in the fanfiction. Dumbledore was highly respected and had a great influence. Knighting Harry would be a way for him to publicly recognize Harry's achievements and heroism on a grand scale.