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.
It could still be Hawk Moth. Since Nathalie has been working with him, getting a Miraculous might make her a threat to his plans, so he could turn against her and become the main antagonist.
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.
One idea could be Nathalie's backstory before she became Gabriel's assistant. Maybe she had a different life goal but got drawn into the Agreste world. Another could be Nathalie dealing with her illness in a more in - depth way, like how she copes when Gabriel is not around to support her. Or perhaps a story where Nathalie discovers a new Miraculous that has an unexpected effect on her relationship with Gabriel and Adrien.
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.
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.