A new dark wizard could emerge as an antagonist. This wizard could be jealous of Harry's power to create an island. He or she might try to steal the magic that keeps the island intact and use it for their own evil plans. Or they could try to enslave the magical creatures on the island for their own gain.
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.
In this fanfiction, Harry could be forced to create an island due to some kind of dark magic threat in the wizarding world. He uses his knowledge and the help of his friends like Hermione and Ron. They might design the island to have its own unique magical ecosystem. For example, they could create a special type of tree that produces potions - ingredients as its fruits. Harry could also make a school on the island for young witches and wizards who are in danger in the regular wizarding world.
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.
He might have discovered the island during one of his adventures and claimed it as his own. Since there was no one else on the island at the time, and he was the first to set foot on it and recognize its potential, he decided to make it his property.
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.
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.
Well, perhaps Harry could use magic to shape the island. He might create a magical forest filled with unique creatures from the wizarding world. He could also build a hidden school on the island to teach magic in his own way, away from the prying eyes of the Ministry of Magic.
Perhaps he uses the island as a secret training ground. He could practice advanced spells away from the prying eyes of Hogwarts faculty and students. Harry might also build his own magical fortress on the island for protection and privacy.
The pure - blood supremacists could be antagonists. They might not like the idea of Harry, a half - blood, inheriting Hogwarts and try to undermine his authority.
It could be the remnants of the Death Eaters. Even after Voldemort's fall, some of them might still be causing trouble and Harry Potter has to bring them to justice for their past and present crimes.
The antagonists could be a group of corrupted Ministry of Magic officials. They have turned to the dark side for power and are trying to rewrite the laws to suit their evil plans. Harry, being the hero, sings 'This is War' to expose their wrongdoings and fight against their tyranny.