Well, this kind of fanfiction could start with Marinette making a mistake while using the Miraculous. Then she finds herself in the form of a Kwami. She might be really tiny and have to find ways to get around. She could hitch rides on people's shoulders or use her small size to her advantage when it comes to spying on Hawk Moth. And she would definitely have to deal with the fact that she can't just transform back into her normal self right away, leading to all sorts of comical and tense situations.
One interesting plot could be that Marinette - as - Kwami is accidentally discovered by Hawk Moth. He tries to capture her to use her powers for his evil plans. Marinette then has to use her wits and any new Kwami - like abilities she has to escape and warn Ladybug (who she can't transform into at the moment). This would create a lot of tension and action in the fanfiction. Also, it could explore how Marinette feels being in such a vulnerable state.
Maybe the Wolf Kwami has a special power that is different from the other Kwamis. A new villain wants to steal this power for their own evil plans. The story could focus on Ladybug and Chat Noir protecting the Wolf Kwami and its power. This would involve a lot of action and adventure scenes.
Let's make a Kwami named Luma. Luma could be a Kwami related to stardust. Its Miraculous could be a star - shaped hairpin. The superhero with this Miraculous would be able to fly faster than normal superheroes, and also have the ability to create a shield made of stardust that can protect from most attacks.
Another way is to have the Kwami OC interact with the existing Kwamis in the Miraculous Ladybug universe. If your Kwami OC is Ember, which has the power of fire, it could meet Tikki and Plagg. They could have a conversation about how their powers are different yet related. This kind of interaction can add depth to your fanfiction.
One possible scenario could be that Marinette gets injured during a fight as Ladybug and is rushed to the hospital. In the fanfiction, her friends might come to visit her, and there could be some sweet moments between her and Adrien as he's worried about her. Maybe she has to keep her identity a secret even while in the hospital, which adds an extra layer of tension.
There's also 'The Luka - Marinette Connection'. In this fanfiction, the connection between Luka and Marinette is explored on a deeper level. It starts with Luka noticing Marinette's little quirks and how she tries to hide her feelings. As the story progresses, they start to open up to each other more. The author creates a really engaging story by adding in some mystery elements related to the Miraculous and how it might affect their relationship.
One great fanfiction could be about Marinette getting the Peacock Miraculous in a unique situation. For example, in a world where all the Miraculouses are scrambled among the characters, Marinette discovers the Peacock Miraculous while on a solo adventure. She has to figure out how to use its powers, which are different from the Ladybug Miraculous she's used to. This could lead to some really interesting character development as she tries to master a new set of abilities.
One possible fanfiction could be about Marinette transforming into a mouse - themed Miraculous holder. Maybe she discovers a new mouse - themed Miraculous in a hidden part of Paris. This could lead to her having new powers like super - speed and the ability to fit into small spaces. She could use these powers to outwit Hawk Moth's akumas in a really unique way, and her relationship with Adrien might also take an interesting turn as she tries to keep her new identity a secret from him.
In Miraculous Ladybug fanfiction, Marinette in a bikini could be part of a beach - themed story. Maybe she's on vacation with her friends. It could be a fun and light - hearted scenario where she shows off her confidence and style while enjoying the sun and sand.