Another trope is the self - rescue. Marinette uses her intelligence and hidden skills to break free from her captors. This shows her strength as a character.
One idea could be that Hawk Moth is behind the kidnapping. He might have discovered Marinette's identity as Ladybug or suspect her. He could use her to try and get the Miraculous. Maybe he offers a trade to Chat Noir, the Ladybug Miraculous for Marinette's safe return.
One interesting 'Marinette kidnapped' fanfiction could be where she is kidnapped by an akumatized villain. In this type of story, the other characters like Chat Noir would have to go on a wild rescue mission. They might face various obstacles and challenges along the way, making for an exciting plot.
If Marinette was kidnapped, Tikki could be a key factor. She might try to communicate with other Kwamis to get help. Since the Kwamis have some special powers and knowledge, they could work together to find Marinette. For instance, Plagg could use his Cataclysm in a unique way to break through any barriers that are keeping Marinette captive.
Marinette could use her wits. If she's tied up, she might be able to find a sharp object nearby to cut the ropes. For example, if she's in a room with some old furniture, there could be a nail or a broken piece of wood that she can use.
Another reaction could be that Marinette pretends to cooperate with Adrien at first. By doing this, she can buy time and look for an opportunity to escape or get help. She might act as if she is okay with what is happening while secretly planning her next move, like leaving a clue for someone to find her or finding a way to disable Adrien temporarily.
In different Miraculous fanfiction, Marinette can be portrayed in various exciting ways. Sometimes she might have new powers or face different challenges compared to the original series. For example, in some fanfics, she could form unique partnerships with other Miraculous holders that we don't see in the show.
She might befriend Iron Man first. Since he's into technology and Marinette is quite creative when it comes to making things. They could bond over creating new gadgets or improving existing ones.
It could be Bruce Wayne himself. Marinette might admire his business acumen and his philanthropy work in Gotham. His mysterious persona as Batman could also be very appealing to her, especially if she has a taste for adventure and mystery.