There could be some old - fashioned professors at Hogwarts who are against Harry's unorthodox ways of making his mark. They believe in the old - established ways of teaching and practicing magic and see Harry as a troublemaker who is disrupting the order of things at the school.
Maybe Harry discovers a new and unique magic that only he can perform. This new magic could be the mark he makes. For example, he might find a way to combine different spells in an unprecedented way that gives him an edge in fighting the dark forces.
It could be Draco Malfoy. He might not like the idea of Harry getting a second chance at the OWLS and tries to sabotage him. Maybe he spreads rumors about Harry not being worthy of a retake.
Hermione could be there. She would be really helpful in planning and organizing the island. For example, she might set up a system for categorizing all the magical knowledge and resources on the island.
It could be a group of dark wizards who are jealous of Harry's status and see Lemon as a way to get to him. They might try to kidnap her or manipulate her to get Harry to do their bidding.
One possible antagonist could be a corrupt wizard in a high - ranking position in the Ministry of Magic's financial department. This wizard might be trying to cover up his misdeeds and would see Harry as a threat to his scheme.
Voldemort's remaining followers could be antagonists. They might target Harry's little sister thinking they can use her to get to Harry. They could try to kidnap her or use dark magic on her.
Scarecrow could also be a main antagonist. His fear - inducing toxins could be a real problem for Harry. Harry would have to use his magic to counter the effects of the fear toxin and also figure out a way to stop Scarecrow from spreading it all over Gotham.
Voldemort or his remaining Death Eaters could still be the main antagonists. They might try to take advantage of Harry's Two - Face situation. For example, they could try to manipulate his 'bad' side to turn against his friends and the wizarding world.
In this fanfiction, there might be a group of wizards who oppose the idea of mixing magic with the skills taught by Hephaestus. They could be the antagonists as they try to prevent Harry from using his new - found skills. They might believe that this new form of magic is dangerous or against the traditional ways of the wizarding world. They could form an organization to oppose Harry and try to disrupt his training and any plans he has to use his skills for good.
It could be the Death Eaters who see Harry's new - found mastery of darkness as a threat to their own power and ideology. They might try to either recruit him or destroy him. Voldemort himself could be an antagonist, feeling that Harry is encroaching on his territory as the most powerful dark wizard.