Hawkmoth could be the main antagonist. In the original story, he's always causing trouble. In a 'marinette dies' fanfic, he could finally succeed in his evil plan, maybe by getting his hands on the Ladybug and Cat Miraculouses and using their combined power against Marinette.
Another plot could involve time - travel gone wrong. Marinette tries to go back in time to fix a major mistake, but something disrupts the time - space continuum and she gets caught in a loop that ultimately leads to her death.
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.
Marinette could react with disbelief and assume it's some sort of joke at first. But when she realizes Adrien is serious, she might start to get really nervous. She could start fidgeting with her hands or her hair. If she's in the middle of doing something, like working on a design, she might drop her tools in surprise. Once the shock wears off, she would most likely be very happy and accept his invitation, but it would be a very flustered and excited acceptance.
Marinette might develop more empathy. Going through pregnancy gives her a new perspective on life. She may understand the struggles of others better, like those who are vulnerable or in need of protection, which could influence how she acts as Ladybug.