In a Sirius Black kidnaps Harry fanfic, perhaps the plot involves Sirius believing that Harry is in immediate danger from Dumbledore. Sirius might think that Dumbledore's decisions are putting Harry at risk, so he kidnaps Harry to keep him safe. He could take Harry to a hidden location, like an old Black family property that has been long forgotten. There, they might discover secrets about Harry's family that Dumbledore has been hiding. This could lead to a rift between Harry and Dumbledore when he eventually finds out. Or, it could be that Sirius has a vision of Harry's future and the only way to change it is to keep him away from the normal path he's on at Hogwarts.
Well, it could be that Sirius Black kidnaps Harry because he thinks Harry is the key to bringing back his best friend, James. He might have found some dark magic ritual that he believes requires Harry's presence. However, as they go through this journey, Sirius realizes that he's wrong and that he's putting Harry in great danger. So, he then has to figure out a way to reverse what he's done and get Harry back to safety. Maybe along the way, they encounter some magical creatures that help them understand the error of their ways. Or they meet some old allies of Sirius who convince him to let Harry go and fight against the real enemy, which is Voldemort and his Death Eaters.
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 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.
One possible reason could be Sirius believing he can protect Harry better on his own. Maybe he thinks the Dursleys are too dangerous for Harry and that the wizarding world isn't doing enough. So, he decides to take matters into his own hands and kidnap Harry to keep him safe.
One possible plot twist could be that Sirius Black, being a wanted man, has to constantly move with Harry to avoid capture. This would make Harry's childhood very different from the one at the Dursleys. He'd grow up learning about the wizarding world on the run, perhaps meeting some unique characters in hiding places.
One possible main plot twist could be that Sirius manages to completely hide Harry from the wizarding world's prying eyes until he comes of age. This would mean Harry grows up without the constant threat of Voldemort's followers finding him. Another twist could be that Sirius discovers some long - hidden magic within Harry that was dormant because of his upbringing with the Dursleys, but flourishes under Sirius's care.
In some fanfictions, Bellatrix might kidnap Harry with the intention of turning him to the dark side. She could try to corrupt him by showing him the power and 'glory' of the dark magic she practices. This would be a huge psychological battle for Harry as he has always been firmly against the dark arts.
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.
One possible plot twist could be that Tom Riddle discovers Harry has a hidden power that he didn't expect. Maybe Harry has the ability to resist Riddle's mind - control spells in a unique way. This could lead to a battle of wills between them where Harry tries to outwit Riddle while being held captive.
At first, Harry would likely be confused and scared. But this experience could force him to become more independent. He can't rely on Dumbledore or his friends at Hogwarts immediately. He has to learn to trust his own instincts and the bond he has with Sirius. This might make him more cautious in the future when it comes to trusting adults. For example, he might question Dumbledore's motives more often.
Perhaps a plot twist could be that Draco is under the influence of a dark spell when he kidnaps Hermione, and as she tries to break the spell, she uncovers a conspiracy within the wizarding world. Or it could be that the kidnapping is a ruse set up by Hermione herself to infiltrate the Slytherin group, but Draco doesn't know it and things spiral out of control.
One possible plot twist could be that the marriage law is part of a dark plot. Maybe a powerful dark wizard has enacted it to create chaos. Harry and Sirius, instead of simply accepting it, start to investigate. They discover that the law is linked to an ancient prophecy that could bring either great good or great evil depending on how they handle their forced marriage situation.