At first, Harry would likely be shocked and angry. He's used to being in dangerous situations, but being kidnapped by Fleur, someone he thought was an ally, would be a big betrayal. He'd probably try to fight back immediately, using his magic if he could.
One possible plot could be that Fleur has been influenced by some dark magic or a powerful enemy. Maybe she's being blackmailed to kidnap Harry. She might take him to a hidden location, perhaps an old, abandoned castle. Then, the story could revolve around Harry trying to figure out why Fleur did this and how to escape while also dealing with Fleur's internal struggle about what she's done.
Yes, there might be such fanfics out there. Fanfic writers often come up with all sorts of creative and sometimes out - of - the - box scenarios. You can try searching on popular fanfic websites like Fanfiction.net or Archive of Our Own. Just type in the title keywords and see what pops up.
Perhaps Fleur is being controlled or influenced by a dark force. In the Harry Potter universe, there are many dark magic entities that could manipulate someone into kidnapping Harry. It could be that she is under the Imperius Curse or some other form of mind control. Another reason could be that she has some sort of personal vendetta against Harry or those close to him, and kidnapping him is her way of getting revenge. She might think that Harry has wronged her in some way, whether it's real or just in her deluded perception.
I haven't come across such a 'fleur kidnaps harry fanfic' directly. However, if you join some Harry Potter fanfic communities on platforms like Reddit or Discord, you could ask the members there. They are often very knowledgeable about various fanfic scenarios and might be able to point you to the right ones.
Maybe Fleur has some ulterior motive. She could be under the influence of a dark wizard who forces her to kidnap Harry. Or perhaps she believes Harry has something she needs desperately, like a powerful magical item that she thinks can save her family or something.
Often, Luna Lovegood can be found in such fanfictions. Her unique and dreamy nature could bring a different dynamic to the harem. Neville Longbottom might also be involved in some way, perhaps as a friend who supports the harem and helps them out in times of need. There could also be some foreign exchange students or magical creatures in human form that end up in the harem for various reasons.
At first, their relationship might be a bit strained. Harry is used to his own space and friends, and Fleur has her own circle. But as they live together, they'll start to respect each other's differences. For example, Fleur might be more flamboyant in her magic use, while Harry is more practical. They'll start to see the value in each other's approaches and become closer friends.
One possible reason could be that Fleur sees Harry as a powerful and important figure. In the wizarding world, Harry is the famous Boy Who Lived. Fleur might be possessive because she wants to be associated with his fame and power. Also, his bravery and skills could be attractive to her, making her want to have him all to herself in the fanfiction world.
Hermione would probably be a bit skeptical at first. She might worry that Harry is rushing into things. But once she sees how happy they are, she would be supportive. Ron, on the other hand, would be more accepting from the start, especially if there are good feasts at the wedding.