Hermione could be a major character. She might use her intelligence to try and figure out where Tom has taken Harry. She could be researching old spells or looking through Tom's past hideouts to find clues.
Another plotline might involve Tom believing that Harry has some hidden power that he can extract or use for his own dark purposes. So, he kidnaps Harry to figure out how to access that power. This could lead to a series of attempts by Tom to break Harry's will and get him to reveal the secret. For example, he might put Harry in a magically sealed room and use various spells to try and force the information out of him. Harry, on the other hand, could be trying to resist and find a way to escape or signal for help.
Harry would likely be defiant. He has faced Voldemort (Tom Riddle) before and he's not one to cower easily. So, he'd probably start looking for ways to escape right away, using his wits and any small magical items he might have 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.
Harry would probably be very defiant at first. He has faced Voldemort (Tom Riddle) before and he won't go down without a fight. He would likely try to use his magic skills to escape or at least hold Tom Riddle off until he can find a way out.
The themes could also include redemption. After kidnapping Marco, Tom might realize the error of his ways and try to make amends. And there could be a theme of friendship. Even though Tom has kidnapped Marco, the story could explore if their friendship can be salvaged.
I'm not sure specifically which 'tom kidnaps marco fanfiction' you are referring to. There could be many different interpretations and stories with such a premise in the fanfiction world. It might involve a complex plot where Tom, for some reason, decides to kidnap Marco. Maybe it's due to jealousy, or a misunderstanding in their relationship within the context of the original story it's based on.
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.
Harry is known for his courage. So, when Tom Riddle kidnaps him, this courage would not let him give in easily. He would face Riddle head - on, not showing any fear, which might unnerve Riddle to some extent. Also, Harry is very loyal to his friends and the wizarding world. This loyalty would give him the strength to resist any offers or threats from Riddle. For example, if Riddle tries to persuade Harry to join the dark side, Harry's loyalty would make him firmly reject such an idea.
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.