One idea could be an OC who has a unique Miraculous that is related to a different aspect of nature, like a firefly Miraculous. The OC could struggle with controlling its powers at first. Another idea is an OC who is from a different country and brings in new cultural elements to the Miraculous world. For example, they might have a special fighting style based on their native martial arts.
Another plot could involve an OC from a different country who comes to Paris with a Miraculous. This OC has different fighting styles and cultural beliefs that add new elements to the story. They could teach Ladybug and Chat Noir about their own Miraculous lore.
One idea could be a crossover with the DC universe. Ladybug and Cat Noir could team up with Superman to stop an akumatized Lex Luthor who has somehow gotten his hands on a Miraculous. Another plot could involve a crossover with the Harry Potter world. Maybe Hawk Moth tries to steal magic from Hogwarts, and Ladybug and Cat Noir have to work with Harry and his friends to stop him.
For a Miraculous Ladybug villain OC fanfic, consider an OC called the Sound Shredder. This villain can emit ultrasonic waves that can shatter glass and disrupt Ladybug and Chat Noir's communication. He was a former musician who lost his hearing due to an accident and now wants to make the world feel his pain. Also, the Frost Fang OC could be a great addition. She has the power to freeze everything in her path. Her backstory could be that she was left out in the cold, literally, by her friends and now seeks revenge on the whole city.
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 the cats in the Miraculous Ladybug world are somehow affected by a new curse that makes them lose their agility. This would cause problems for Cat Noir as he tries to fight villains. Another could be a curse that makes all the cat - related items in Paris turn into something else, like the catacombs becoming a maze of thorns, and Ladybug and Cat Noir have to find a way to reverse it.
One plot idea could be that Ladybug and Chat Noir have a huge fight due to a misunderstanding, which leads to a 'hate' phase. But as they face a powerful villain alone, they realize they need each other and slowly start to rekindle their friendship and the underlying love. For example, Chat Noir could be wrongly accused by Ladybug of working with a new villain. As they work separately to defeat the villain, they see the struggles each other faces and remember why they were partners in the first place.
Maybe Marinette goes to a summer art camp and Adrien is also there. They work on a big art project together and their feelings for each other grow as they share their creativity and passion for art. During the summer nights, they stargaze and have deep conversations.
One idea could be a story where the Peacock Miraculous has a unique malfunction that causes it to create unexpected akumas. For example, instead of creating the usual villains, it creates akumas that have the power to spread positive emotions but in a chaotic way that still disrupts Paris. Another idea is a fanfiction where the holder of the Peacock Miraculous has to team up with Ladybug and Chat Noir to fight against an ancient evil that is specifically drawn to the power of the Peacock. This could involve exploring the history of the Miraculous and its connection to this new threat.
One idea could be Multimouse having to team up with an unexpected ally, like a villain who has a change of heart for a short time. Another could be Multimouse getting lost in an alternate dimension and having to find her way back while facing new challenges.
One idea could be Ladybug and Chat Noir having a heart - to - heart conversation in bed after a particularly tough battle. They could share their fears and insecurities that they usually hide behind their superhero personas.