One common theme is the discovery of their true identities. Since they don't know each other's secret identities in the show, a lot of fanfiction focuses on how they find out about each other. Another theme is their relationship development, from friends to something more romantic.
A popular theme is also the 'what - if' scenarios. For example, what if they got stuck in an alternate dimension together? Or what if they had to fight a villain without their Miraculous? These types of scenarios add an interesting twist to the story. Also, there are many fanfictions that explore their insecurities and how they overcome them together, which is really heartwarming.
One interesting Marinette and Cat Noir fanfiction could be where they have to face a new villain that has powers they've never encountered before. In this story, Marinette might struggle with her identity as Ladybug while also dealing with her feelings for Cat Noir. Another could be a time - travel fanfic, where they go back in time and meet their predecessors as superheroes.
One popular scenario could be during a battle where they are about to be defeated and in a moment of desperation and realization of their true feelings, they share a kiss. This kiss might give them a sudden burst of power to defeat the villain. Another could be at a school dance. Marinette is hesitant at first but Cat Noir sweeps her off her feet and they kiss under the twinkling lights. And there's also the scenario where they are alone on a rooftop, looking at the city, and the atmosphere just leads to a tender kiss.
There's also 'A Kiss in the Rain' which is quite well - loved. In this story, the setting of the rain adds a lot of romantic tension. It shows how their relationship develops to the point of that kiss in a very natural way. It also has some really sweet dialogue between Cat Noir and Marinette that makes the kiss even more meaningful. The story goes into their insecurities and how they overcome them to reach that moment of closeness.
Friendship is also a big theme. Through these fics, their friendship deepens as Chat Noir comforts Marinette. It shows how they can rely on each other even outside of their superhero and civilian identities.
Another storyline could involve a situation where they are forced to hide their feelings in public but when they are alone, like in an abandoned building that they've made their secret hideout, they engage in making - out. It adds an element of secrecy and excitement to the story.
No, it is not appropriate. 'Miraculous Ladybug' is a popular animated series aimed at a wide audience including children. Writing sexual fanfic about the characters goes against the family - friendly nature of the show and can be considered inappropriate and disrespectful to the original work and its fans.
You can start by looking on fanfiction websites like Archive of Our Own (AO3). It has a large collection of Marinette and Cat Noir fanfictions, with different tags to help you find exactly what you're looking for, whether it's a specific genre like romance or action, or a particular plot type. Another option is FanFiction.net, which also has a decent number of stories in this fandom.
Write about their friendship development. Maybe explore how they gradually become better friends outside of their superhero duties, like having normal conversations or sharing hobbies. It can add depth to their relationship in a positive and appropriate way.
The power struggle is also a theme. Gabriel as Hawkmoth has power, and Cat Noir has his own powers. Fanfics might explore how they use or misuse their powers in relation to each other. For example, when they face off in battles, the power dynamics are constantly shifting.
Another theme is growth. The little sister often grows from being an innocent bystander to someone who is actively involved in the fight against evil. This could involve her overcoming her fears and learning new skills, like how to use a special weapon or gadget.