Maybe Hawk Moth himself decides to target the ice - skating community. He could akumatize a skating judge who then gives unfair scores to the skaters. Ladybug and Chat Noir would have to deal with this unjust situation and stop the akumatized judge from causing more chaos in the skating world.
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.
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.
It could also be Lila. In this au, she could be a master manipulator who uses her lies to create chaos among the heroes and the citizens. She teams up with other criminals and uses her charm to get away with various crimes while making Ladybug and Chat Noir look bad. She might even be able to get her hands on some sort of powerful weapon or item that gives her an edge over the heroes.
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.
Hawk Moth would be an obvious choice as he is the main antagonist in the original series. He could create an akuma specifically designed to target Ladybug when she is in a wet or water - related situation. Maybe an akuma that can control water currents and use them to attack Ladybug, making her 'wet for me' part of the akuma's strategy to weaken or distract her.
One idea could be Ladybug and Chat Noir having to skate to defeat an akumatized ice skater villain. They use their powers in unique ways on the ice, like Ladybug using her yo - yo to create ice - skating ramps. Another plot could involve a skating competition where Marinette (Ladybug) is competing and has to overcome self - doubt, all while keeping her identity a secret from the other skaters.
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.