Here are some plot ideas. Felix and Marinette could be paired up for a dance competition at school. At first, they are both reluctant, but as they practice, they develop feelings for each other. Another idea is that there's a new villain in town who targets Marinette specifically because of her connection to the Miraculous. Felix, being the observant person he is, notices the strange things happening around Marinette and decides to help her. He might even pretend to be her bodyguard. And finally, a story where Felix and Marinette accidentally swap bodies due to a magical mishap. They have to figure out how to reverse it while also dealing with each other's lives and responsibilities.
One interesting plot could be Felix discovers Marinette's secret identity as Ladybug by accident. He then has to decide whether to use this information for his own gain or keep it a secret. Maybe he starts to develop feelings for her because of her dual identity and the strength she shows as Ladybug.
One plot idea could be that Marinette and Felix are paired up for a school project. Felix, initially cold, starts to see Marinette's kindness and creativity, and they develop an unexpected friendship. This could lead to some comical moments as Marinette tries to hide her Ladybug identity from him while Felix is being his usual perceptive self.
She could be turned into a mouse - like creature by an experimental akuma attack. As a mouse, she discovers new parts of Paris that are usually hidden from humans. This could include secret underground tunnels or the rooftops in a whole new perspective. She might also meet other kwamis who are more active in these hidden areas.
One plot idea could be Marinette initially struggling to control the power of the Butterfly Miraculous. Since she's used to the Ladybug Miraculous, the different nature of the Butterfly's power might be overwhelming. For example, she might accidentally create akumas with stronger powers than she intended. Another idea is that she could use it to try and understand Hawkmoth better. By having the Butterfly Miraculous, she could explore the emotions that drive people to be akumatized from a different perspective. And perhaps she teams up with Chat Noir in a new way, as they both have to adapt to her new power source.
One plot idea could be Marinette having to deal with the pressures of being a princess while also being Ladybug. For example, she might have to attend royal balls and diplomatic events in her princess role, all while secretly protecting Paris as Ladybug. Another idea is that an evil villain targets her specifically because of her dual identity, trying to take over both the kingdom and Paris. Maybe there could also be a plot where she has to choose between her duties as a princess and her responsibilities as Ladybug, creating an internal conflict.
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.
One idea could be exploring how Marinette balances her superhero duties with her pregnancy. Maybe she has to be more cautious during battles, and her partner Chat Noir has to be extra protective. Another plot could involve her friends and family finding out. Some might be overjoyed while others could be worried about how she'll handle it all.
One idea could be that Marinette vomits due to extreme stress. Maybe she's facing too many akuma attacks in a short time and the pressure gets to her physically. Another could be related to a strange side - effect of using the Ladybug Miraculous. For example, if she overuses her powers in a short period, it might cause some sort of physical reaction like vomiting.
A possible plot idea is that Marinette is akumatized after being wrongly accused of something at school. Her akumatized form could have the power to manipulate people's memories, making them believe false things about their friends. This would cause a lot of chaos and mistrust in Paris. Chat Noir would have to find a way to reach the real Marinette inside the akumatized shell and convince her to break free. It would be a story full of drama and action.
One common plotline is the 'enemies - to - lovers' trope. Felix might start off as cold or indifferent towards Marinette, but through a series of events, they end up kissing. For example, they could be forced to work together on a school project or to save the day in the Miraculous - filled world, and during that time, their feelings gradually change.