Maybe in a 'Harry kidnaps Ginny' fanfic, it's all a big misunderstanding. Harry thinks Ginny is in danger from some other dark force, but his way of 'protecting' her looks like kidnapping. Ginny is initially scared and angry but then realizes Harry's intentions and they work together to face the real threat.
One possible plot could be that Harry is under the influence of a dark spell. In this state, he kidnaps Ginny because the spell is making him act against his true nature. Ginny, being the strong - willed character she is, tries to break through to Harry and reverse the spell's effect on him.
It could be that an evil entity has possessed Harry. This entity wants to use Ginny as a bargaining chip against the Weasley family or the Order of the Phoenix. Harry, while being controlled, kidnaps Ginny. Ginny, however, uses her knowledge of magic and her connection with Harry to fight off the possession and free them both from the dangerous situation.
One possible plot could be that Harry is under the influence of a dark spell or some mind - controlling magic that makes him kidnap Ginny. Maybe he's been hexed by a powerful dark wizard who wants to use Ginny as a bargaining chip against the Order of Phoenix. So, Harry, without his true self in control, takes Ginny to a hidden location.
It could be that they are on a mission together and get poisoned by some magical substance that has the side effect of making them drunk. In their inebriated state, they start reminiscing about their past, from their first meetings to their first kisses. This trip down memory lane makes them realize how much they truly love each other and how far they've come as a couple.
One possible plot twist could be that Tom Riddle kidnaps Harry not for the typical reasons like revenge or to gain power over the wizarding world. Maybe he discovers that Harry has some hidden knowledge about a long - lost magic that he wants for himself. Another twist could be that while kidnapped, Harry manages to find a way to get into Tom Riddle's mind and discovers his deepest fears, which he then uses to his advantage to try and escape. It could also be that there are other dark forces at play that Tom Riddle is actually trying to protect Harry from, in his own twisted way, by taking him away from the wizarding world for a while.
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.
One possible plot twist could be that Sirius Black has a noble reason for 'kidnapping' Harry. Maybe he discovers a plot against Harry within Hogwarts that the school's authorities are ignoring. So, he takes Harry away to protect him. For example, he might find out that a certain teacher is under the influence of Voldemort and plans to harm Harry. Another twist could be that Sirius Black is being controlled by a dark magic spell and doesn't really mean to harm Harry. It could be a setup by Voldemort's followers to make Sirius look bad. Also, it could turn out that the 'kidnapping' is part of a secret training plan for Harry. Sirius wants to train Harry in some special magic skills away from the prying eyes of Hogwarts.
One possible plot could be that Tom Riddle, using his charm and manipulation skills, lures Ginny into a secluded place under false pretenses and then takes her captive. He might be after some information that he believes Ginny has, perhaps related to the secrets of Hogwarts or the Weasley family. Ginny, being a brave character, would constantly try to resist and find a way to escape while Tom Riddle would be using his dark magic to keep her in check.
One possible plot could be that they initially panic. Harry might worry about being able to protect his new family while Ginny would be thinking about how this would change their lives. They could face reactions from their friends and family, some being supportive and others expressing concern. Then they start to prepare for the baby, like finding a new place to live that's more suitable for a family.
In such a fanfic, perhaps Ginny has been feeling neglected by Harry due to his constant focus on fighting dark forces or some other important matters. So she starts to seek comfort in someone else. When Harry finds out, he first feels angry and betrayed. But then he realizes that he also had a part to play in their relationship going sour. He could then try to win Ginny back or decide to move on, depending on how the author wants to develop the story.
Maybe Ginny gets a fever after being exposed to a dangerous magical artifact. The fever makes her have strange dreams. Harry, being the hero, tries to enter her dreams to figure out what's wrong. He battles the dark forces within her dreams and finally manages to wake her up and cure her fever.
Well, one possible plot could be their daily adventures in raising their children. Maybe they face some magical challenges in protecting their family from dark forces that still lurk in the wizarding world. They could also be involved in rebuilding Hogwarts and passing on their magical knowledge to the new generation.