It could be the Mad Father himself. If he were to find out about the Miraculous and try to use them for his own twisted experiments, he would make a great antagonist. He could try to capture Ladybug and Chat Noir to study their powers.
Aizen could be a great main antagonist. He's extremely intelligent and powerful in the Bleach universe. He might see the Miraculous as a means to further his own goals, whether it's to gain more power or to create more chaos. His manipulative nature would make him a formidable foe against Ladybug and Ichigo.
It could be a new villain who has a power specifically related to countering the ribbon. For example, a villain who can create anti - ribbon energy that weakens Ladybug's abilities when she uses the ribbon. Maybe this villain is jealous of Ladybug's power and wants to take the ribbon for themselves to become the most powerful in Paris.
Maybe an old enemy of Ladybug or Chat Noir who sees Bluerosesnow as an easy target to get to them. They could use her as a pawn in their evil plans, trying to manipulate her into doing things that would harm the heroes. This would create a lot of conflict and drama in the fanfiction.
It could be Hawkmoth. He might see the zombie chaos as an opportunity to get the Miraculouses. He could manipulate the zombies or even try to create a super - zombie under his control to defeat Ladybug and Chat Noir.
Maybe an akumatized version of a weather forecaster or a meteorologist. Frustrated by always being wrong about the weather or not being taken seriously, Hawkmoth akumatizes them. They then use their knowledge of the weather and new - found powers to create the perfect storm and cause havoc in Paris, making them a formidable antagonist for Ladybug and Chat Noir.
It could be a Miraculous holder who has been corrupted by the power. They might start to think that they should be the sole leader and use the civil war as a means to take over. For example, if a holder of a powerful Miraculous like the Bee Miraculous turns bad, they could be a formidable antagonist.
It could be Lila. Since she is always causing trouble in the series, in a Nathalie - centered fanfiction, she could target Nathalie as well. Maybe she tries to frame Nathalie for something to get closer to Gabriel or to get rid of Nathalie who she sees as an obstacle to her plans.
Chloe could be the main antagonist. She has a history of being jealous of anyone getting close to Adrien, especially Marinette. So, she might set a trap for Adrien out of spite or to keep him all to herself.
Gabriel could still be an antagonist even in a baby Adrien fanfiction. He might be trying to use baby Adrien's innocent charm to get closer to the Miraculous secrets without Adrien realizing it.
It could be a new character, like a dark sorcerer who wants to steal all the Miraculouses for his own dark magic. He could be specifically targeting the Dark Owl because he sees the Dark Owl as a threat to his plans.