It could center around the son of a villain who discovers a miraculous of his own. This miraculous might be different from the ones Ladybug and Chat Noir have. He then has to learn how to use it while also dealing with the fact that his family is against the heroes. His journey could be about finding his own identity, whether he wants to follow in his family's footsteps or become a hero in his own right. He may also face challenges from other villains who don't trust him because of his family ties.
Ladybug and Chat Noir would likely be main characters as well. Since it's a Miraculous Ladybug fanfiction, they are important figures in that universe. They would interact with the son of the villain, either as enemies at first or as potential allies. And then there could be the villain parent. This character would have a big influence on the son's life and actions, constantly pulling him towards the path of villainy.
To create a great villain for Miraculous Ladybug fanfiction, focus on their flaws and strengths. Make their actions unpredictable and have them interact with the main characters in interesting ways. Maybe they have a hidden agenda or a secret that's revealed later on.
Title: 'The Rise of the Shadow Serpent'. In this fanfic, my villain OC is the Shadow Serpent. She was once a kind-hearted girl, but after being betrayed by those she trusted, she turned to the dark side. She uses her power to create illusions and chaos in Paris. Ladybug and Chat Noir face a tough battle as they try to figure out her motives and weaknesses.
A villain that has the power to steal emotions could be a great choice. Imagine this villain targeting Edendaphne's friends and allies, draining their positive emotions. Edendaphne would have to find a way to not only stop the villain but also restore the emotions of those affected. It would be a very emotional and intense story.
A new character could be introduced as the main villain. Maybe a corrupt police officer who turns to the dark side. He could use his knowledge of the law and the city's security systems to commit crimes and frame the Miraculous heroes. He would be a formidable opponent as he has insider knowledge.
An evil alien warlord could be the main villain. This warlord could be bent on conquering Earth and other planets in the universe. He might see the Miraculouses as a threat and try to destroy Ladybug and Chat Noir first.
Hawkmoth is an obvious choice. His whole mission is to obtain the Miraculouses, and in a death fanfiction, he could succeed in a more devious way than in the original series, directly or indirectly causing Ladybug's death.
It could be a new villain created specifically for the fanfiction. Maybe an ancient sorcerer who was trapped for centuries and now wants revenge on the modern - day heroes. He has the power to turn things to stone and sees Ladybug as the main threat to his plans.
A possible main villain could be a character from Ladybug's past who has discovered her identity and is using that knowledge to create chaos. They could be blackmailing her or trying to steal her Miraculous by setting up elaborate mysteries and traps for her to solve.