She could have been kidnapped by a new villain who wants to use her skills as a fashion designer for some evil purpose. Since Marinette is so talented, this villain might think she can create special costumes or items for their evil plans.
In the fanfiction, Marinette might have gone missing due to a misunderstanding. Maybe she overheard something that made her think she was a danger to her friends, so she ran away. The story could then be about how her friends try to convince her that she is wrong and that they need her, especially with all the Akuma attacks in Paris.
Perhaps Marinette is missing because she has been cursed. The curse could be making her invisible or transporting her to a different time. Her friends would have to find a way to break the curse. Adrien would be very worried as he has feelings for Marinette, and as Chat Noir, he would be determined to save her no matter what.
She could be in the hospital because of a bad fall during a chase in her Ladybug form. Her classmates might start a fundraising campaign to help with her medical bills. And Chat Noir could be secretly visiting her at night, leaving little gifts by her bedside.
In this fanfiction, Marinette and Tikki could be transported to an alternate version of Paris. Here, everything is different, and they have to find a way back. Marinette uses her wits and Tikki's knowledge to navigate this strange new world. They meet different versions of their friends and enemies, and through their adventures, they learn more about themselves and their relationship.
Perhaps there's a beach fashion show in the fanfiction. Marinette enters wearing her bikini and wows everyone with her unique style. It could also be a moment where she catches the eye of Adrien, who is there to support his friends in the show.
In a Marinette - missing fanfiction, it could be that she has amnesia and has wandered off. She might not remember who she is or where she lives. The other characters would notice her absence quickly. Tikki would be especially worried and try to guide Chat Noir to find Marinette. The city of Paris would seem a little less bright without her, and the story could focus on how everyone comes together to find her and restore her memory.
Maybe she was framed by an Akuma - influenced villain. In the 'Miraculous Ladybug' world, this is quite likely. Her arrest could be part of a larger scheme to distract Ladybug and Chat Noir. So, Ladybug might have to balance her superhero duties with clearing Marinette's name, which would be a really interesting plot twist.
One possible plot could be that Marinette enters a singing competition. She is super nervous at first, but as she starts singing, she gains confidence. Her song could be about her journey as Ladybug, without revealing her identity of course. And her performance might attract the attention of a new villain who wants to use her voice for their evil plans.
She might receive a mysterious gift that has some connection to her Ladybug identity. For example, it could be an ancient book about the Miraculous that has been lost for centuries. This gift could lead her on a new adventure to uncover more about the history of the Miraculous and her role as Ladybug.
The plot could involve Marinette trying to hide her new flying ability from everyone at first. She might be afraid of how others will react or that Hawk Moth will find out and target her more. So she practices in secret, which leads to some comical moments like almost getting caught by Tikki.
In a Miraculous Ladybug sleepover fanfiction, the characters might have a lot of fun. They could play games like truth or dare, which could lead to some interesting revelations. For example, Marinette might accidentally let slip some of her feelings for Adrien.
They could be paired up for a hero - training exercise. Since Marinette is Ladybug and Chloe has been Queen Bee in the past. They have to learn to work as a team. This forces them to put aside their differences. They might start to see each other in a new light. For instance, Marinette could learn that Chloe has a lot of knowledge about Parisian high society that could be useful in fighting villains. And Chloe could see Marinette's strategic thinking as Ladybug.