Voldemort could be the antagonist. He might find out that Harry needs Fleur to live and try to capture or kill Fleur to get at Harry. Since Harry is a threat to his power, eliminating the person Harry depends on for his life would be a strategic move for Voldemort.
Perhaps Fleur is a key part of a prophecy related to Harry's life. In this fanfic, it could be that without her presence and active participation in Harry's life, he will die. For instance, she might be the one who can unlock a hidden power within Harry that is necessary for his continued existence. Like she has to be with him at certain times or places to trigger this power.
It could be a rogue dark wizard who has a personal grudge against Fleur. He might see Harry as an obstacle in his path to harm Fleur. So he captures Harry and tortures him, thinking that it will stop Harry from interfering with his evil intentions towards Fleur.
Voldemort's remaining followers could be the antagonists. Even though Voldemort is gone, there are still some Death Eaters who are bitter about their loss. They might see Harry leaving with Fleur as an opportunity to get revenge on Harry or to use Fleur as a hostage to gain power. They could try to ambush them on their journey, creating a lot of dangerous situations for Harry and Fleur.
The exploration of their individual characters within the context of the relationship is also important. Harry's bravery and Fleur's allure are enhanced when they are together. The fanfic may show how Fleur's veela charm affects Harry in a new way, and how Harry's heroism makes Fleur see him in a different light. This character exploration helps to make their relationship more believable and engaging.
One popular 'harry needs to be with fleur fanfic' could be 'The Unforeseen Bond'. In this fanfic, the author weaves a story where Harry and Fleur's relationship develops in the aftermath of the Battle of Hogwarts. They start to discover shared interests and a mutual respect that slowly blossoms into a deeper connection.
It could be Voldemort, even if he's already been defeated in the original story. His lingering influence might still haunt Harry. Maybe there are some dark magic remnants of Voldemort that keep causing problems and making Harry feel hopeless.
One good fanfic could be 'The Fleur's Care' where Fleur uses her Veela charm and healing magic to take care of Harry after a difficult battle. She helps him recover both physically and mentally, and their relationship grows stronger throughout the story.
A new dark wizard or witch could be introduced as the antagonist. This character could be even more powerful and cruel than Voldemort. They could have a special power that allows them to break Harry's spirit, perhaps by constantly torturing his mind or by causing harm to those he loves in ways that are even more insidious than Voldemort's methods.
It could be an archaeologist who has been corrupted by the power of an ancient artifact related to the mummy. This archaeologist wants to use the mummy's power for his own gain, like becoming immortal or ruling the world. He would be constantly trying to stop Harry from helping the mummy.
It could be a powerful dark wizard who has also gained some supervillain - like powers. This antagonist might be jealous of Harry's newfound superhero status and wants to destroy him.