Perhaps a villain who can manipulate time. This would put Carapace in a difficult situation as he would have to strategize around the constantly changing time. It could be someone who was once a failed Miraculous user who turned to the dark side.
There's a fanfiction called 'Carapace Unleashed' which focuses on Carapace's powers and how he discovers new ways to use them. It has an exciting plot with lots of action scenes where he battles against new villains. In this fanfiction, his relationship with other Miraculous holders is also explored in detail, making it a really engaging read.
One idea could be Carapace getting a new power upgrade and having to learn to control it while facing a powerful new villain. Maybe he teams up with a new Miraculous holder.
First, you should study the character of Carapace. Know his powers, his personality traits like his loyalty and his sense of humor. Then, come up with an interesting plot. It could be something like him going on a solo adventure or having to solve a mystery within the Miraculous world.
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.
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.
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.
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.
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 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 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.