It could be a new villain created specifically for the fanfiction. Maybe someone who has a vendetta against Chloe and targets her because she has the Miraculous.
There's also the possibility that the main antagonist is actually an aspect of Chloe herself. Her own insecurities and self - centered nature could be personified as an internal struggle within the fanfiction. So, she has to overcome her own 'dark side' while using the Miraculous to do good.
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.
Maybe an evil artist from another dimension. This artist is jealous of Nathanael's pure creativity and the unique power of his miraculous. They come to Nathanael's world to steal his ideas and use his miraculous for their own dark purposes, like creating chaos and destruction through art.
There could be a group of hellish bureaucrats who are the antagonists. They are the ones who sent Chloe to hell in the first place and are trying to keep her there to maintain the order of hell as they see it. They might not want Chloe to expose the unjust system that exists in hell.
An alien entity that has been dormant beneath Smallville for centuries could awaken. This entity could view Chloe and Clark as threats to its plans of taking over the town and the world. It could use its advanced powers to manipulate the townspeople and turn them against Chloe and Clark, making it a formidable foe.
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 possible plot could be Chloe's journey of self - discovery with the Miraculous. Maybe she starts off using it for the wrong reasons, like for personal gain or to show off, but then realizes the true power is in helping others.
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.