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.
In the fanfiction, Nathanael might get a miraculous related to his artistic talent. It could be a miraculous that enhances his creativity to superhuman levels. He could then use this to create amazing works of art that have some sort of magical or practical effect on the world around him, like inspiring people to be more kind or even physically changing the environment in a positive way.
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 created specifically for the fanfiction. Maybe someone who has a vendetta against Chloe and targets her because she has the Miraculous.
One that stands out is 'Nathanael's Friendship with Ladybug'. This fanfiction explores the friendship between Nathanael and Ladybug in a really heart - warming way. It shows how Nathanael's kindness and his unique perspective can bring something new to Ladybug's life and how they can support each other, which is really appealing to fans.
One great 'Miraculous Ladybug Nathanael Fanfiction' is 'Nathanael's New Beginnings'. It shows Nathanael in a more confident role, exploring his art in new ways while also having some sweet interactions with the other characters. It really delves into his character development and gives him more depth than in the original show sometimes.
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.
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.