There could be a fashion designer from a rival school who is jealous of Marinette's swimwear designs. This character could be a bit of a troublemaker and might unknowingly help Hawkmoth in his plans to steal Marinette's designs. But in the end, she could see the error of her ways and become a friend.
A potential new character could be a magical art teacher who has the ability to bring out the best in Evillustrator's powers. Another could be a street artist who becomes friends with Evillustrator and gets caught up in the superhero - villain drama. Maybe a young aspiring artist who looks up to Evillustrator and tries to help him turn good.
A new villain could be a former Miraculous holder who turned evil due to some misfortune. Maybe they felt betrayed by the Order of the Miraculous and now seek revenge. Another possibility is an alien entity that is attracted to the power of the Miraculouses and wants to use them to conquer Earth.
One interesting new character could be a young girl who has the power to sense when the Reverser is about to use his powers. She could be an ally to Ladybug and Chat Noir, helping them anticipate his attacks. Another could be a mysterious magician who was once affected by the Reverser's powers and now has a vendetta against him. He might have some unique magic that can counter the Reverser's reversals in certain situations. A third new character could be a tech - genius kid who creates gadgets that can detect and temporarily block the Reverser's power waves.
Obviously Ladybug and Chat Noir would be the main characters. They could be joined by other Miraculous holders like Rena Rouge or Carapace as they enter the gaming realm.
A new ice - themed superhero could be introduced. Maybe someone who has the power to control warm air to counter Glaciator's ice powers. This character could be a young scientist who was working on a climate - control project when they discover their powers. Another option could be a villain who is jealous of Glaciator's power and wants to steal it for themselves. They could have a fire - based power which creates an interesting contrast with Glaciator's ice.
Perhaps an outside force, like an alien entity, could invade the hero school. This entity could be attracted by the powerful Miraculous energy. It would be a very different kind of villain as it has no concept of the human values that the heroes at the school uphold, and it would take all of their combined skills to defeat it.
The friends of the next - generation Ladybug and Cat Noir could also be main characters. For instance, the child of Alya and Nino might have a Miraculous too. They could bring in different skills and perspectives. Maybe this character is really into technology because of their parents' love for media and music.
Well, obviously Ladybug and Chat Noir should be in it. They could interact with the cousins and help them learn about their powers. Then there could be the villain characters like Hawk Moth. His presence can create conflict and challenges for the cousins. Another good character could be Master Fu. He can guide the cousins in understanding the deeper meaning and responsibilities of the Miraculous.
Ladybug and Chat Noir would likely be main characters as well. Since it's a Miraculous Ladybug fanfiction, they are important figures in that universe. They would interact with the son of the villain, either as enemies at first or as potential allies. And then there could be the villain parent. This character would have a big influence on the son's life and actions, constantly pulling him towards the path of villainy.
Adrien and Chat Noir could also be the main characters. Adrien is a model with a lot of public image to maintain, and Chat Noir is a flirty and carefree superhero. If they switched, there would be a lot of drama. Adrien might struggle with Chat Noir's more laid - back attitude towards his superhero identity, and Chat Noir would have to deal with Adrien's strict family life.