Maybe some jealous students from Hogwarts or Beauxbatons. They don't like the idea of Harry and Fleur being together and running away to have an adventure. So they try to sabotage their plans, perhaps by spreading false rumors or even using minor hexes to slow them down on their journey.
Voldemort and his Death Eaters would be obvious antagonists. They could be constantly hunting for Harry and Tonks as they are seen as threats to Voldemort's plans for total domination of the wizarding world.
The remaining Death Eaters could be the antagonists. Even though Voldemort is gone, there could be some who still hold a grudge against Harry and want to finish what their master started. They might track Harry and Ginny down as they run away, creating dangerous situations for the couple.
Maybe for love. They might have fallen deeply in love and want to be away from the prying eyes of the wizarding world.
Well, perhaps they run away because of a misunderstanding within the wizarding community. Harry and Fleur might have been wrongly accused of something, like stealing a powerful magical artifact. So they decide to flee and clear their names on their own terms. They journey through different magical lands, encountering various magical creatures along the way that either help or hinder them.
The Ministry of Magic itself could be an antagonist. If Fred and Hermione are running away because they've uncovered some sort of corruption or dark secret within the Ministry, the Ministry officials would want to stop them from spreading the truth.
Sometimes, it could be the pure - blood supremacists. They might not like the idea of Harry, who is a symbol of the fight against them, being betrothed to Fleur, who is part - Veela. They could try to create problems between them.
In this fanfiction, Percy and Thalia could meet a powerful sorcerer who has been exiled from the magical community. The sorcerer could see potential in them and offer to teach them some ancient magic spells that could be useful against their enemies. Additionally, they may come across a group of forest spirits. These spirits could be initially hostile but after learning of Percy and Thalia's noble intentions, they could provide them with shelter and information about hidden paths that lead to safe places away from the threats they are running from.
Harry, the boy who survived Voldemort's attack as a baby and became a key figure in the fight against the Dark Lord. And Daphne, a character who may not be as prominent in the original books but takes on a major role in this fanfiction. Their personalities and how they interact with each other while on the run are what make up much of the story.
Some of Harry's friends could also be antagonists in this fanfiction. They might not understand his reasons for wanting to destroy the wizarding world and try to stop him. For example, Hermione, with her love for knowledge and the wizarding world's institutions like Hogwarts, would probably try to reason with Harry and, if that fails, might even fight against him to protect the world she loves.
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.