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.
Lila could also be a main villain in a criminal fanfiction. She could be using her manipulative skills to start a criminal empire. She convinces others to do her dirty work, and when things go wrong, she's always able to shift the blame onto someone else, like she does in the original series.
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.
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.
It could also be Lila. In this au, she could be a master manipulator who uses her lies to create chaos among the heroes and the citizens. She teams up with other criminals and uses her charm to get away with various crimes while making Ladybug and Chat Noir look bad. She might even be able to get her hands on some sort of powerful weapon or item that gives her an edge over the heroes.
Hawk Moth would be an obvious choice as he is the main villain in the series. He could be extra devious during Christmas, trying to ruin the festive mood for everyone in Paris. He might akumatize people who are feeling left out or jealous during the holidays.
The main villain could be a pirate captain who has found a dark Miraculous. He uses its power to control other pirates and turn them into his minions. He wants to use the power of the Ladybug and Cat Noir Miraculous to become the most powerful being on the seas.
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.
A time - traveler from the future could also be an interesting main villain. This character could have seen a dark future where the Miraculouses were misused and has come back to the present to destroy them all. But in his quest, he doesn't care who gets hurt, and Emma has to stop him from changing the past and future in a bad way.
Hawkmoth could be the main villain as always. He might see the Valentine's Day spirit or all the love in the air during the Love Patrol's time as an opportunity to akumatize someone. Maybe he akumatizes a jilted lover who then goes on a rampage against happy couples in Paris.
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.