Maybe Hawkmoth somehow gets involved with the Mad Father's experiments. Ladybug and Chat Noir have to team up not only to stop Hawkmoth but also to prevent the Mad Father's madness from spreading further. They could face new types of villains created from the combination of the two universes.
Another plot could be that a character from the Mad Father's world stumbles into Paris. Ladybug, thinking they are a new akuma at first, tries to fight them. But then realizes they are just lost and scared. So, she decides to help them find their way back home while also learning about their world and how it could potentially impact her own.
One idea could be that Marinette from Miraculous Ladybug discovers the dark secrets of the Mad Father's world while on a school trip. She uses her Ladybug powers to protect herself and those around her from the horrors there.
One idea could be that Ladybug and Chat Noir get transported to the Undertale universe. They might have to team up with Sans and Papyrus to fight against a common enemy that is threatening both their worlds. Maybe the akumas in the Miraculous Ladybug world are somehow related to the dark forces in the Undertale world.
One idea could be a story where the Peacock Miraculous has a unique malfunction that causes it to create unexpected akumas. For example, instead of creating the usual villains, it creates akumas that have the power to spread positive emotions but in a chaotic way that still disrupts Paris. Another idea is a fanfiction where the holder of the Peacock Miraculous has to team up with Ladybug and Chat Noir to fight against an ancient evil that is specifically drawn to the power of the Peacock. This could involve exploring the history of the Miraculous and its connection to this new threat.
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.
One idea could be that the Reverser's powers are amplified and he starts reversing not just individual actions but entire relationships in Paris. So, Marinette and Adrien's unspoken bond is reversed, making them avoid each other at all costs. Another could be that the Reverser targets Ladybug's and Chat Noir's memories of their superhero identities, causing chaos as they try to figure out who they are again while still having to fight akumas. A third idea is that the Reverser somehow manages to reverse the concept of good and evil in Paris, making the akumas seem like the heroes and the heroes like the villains, and Ladybug and Chat Noir have to find a way to reverse it back.
One idea could be that the thief has a secret noble cause. Maybe they steal to return precious items to their rightful owners who were wronged in the past. For example, an artifact was stolen from a family long ago, and the thief is trying to make things right.
One idea could be that Zombizou actually has a hidden power within her zombie state that she can transfer to Ladybug or Chat Noir to help them defeat a new super - villain. For example, she might be able to make them invisible to the villain's special powers for a short time.
One idea could be a story where the child of Ladybug and Chat Noir discovers their parents' secret identities by accident. Maybe they find a hidden Miraculous box in the attic and start to figure out what it means.
A possible plot for the Miraculous Ladybug Glaciator fanfiction is that Glaciator somehow freezes time in Paris, except for Ladybug and Chat Noir. They have to find a way to reverse the time - freeze while also dealing with Glaciator's attacks. Or perhaps Glaciator teams up with another akumatized villain, creating a more difficult situation for our heroes. Also, the fanfiction could explore the relationship between Ladybug and Chat Noir as they are trapped in this frozen - in - time Paris, and how they grow closer or face new challenges in their relationship.
One idea could be exploring the emotional aftermath for the characters after the events of Puppeteer 2. For example, how Ladybug and Chat Noir's relationship might have changed. Another could be introducing a new villain who takes advantage of the chaos left behind. Maybe a new Miraculous holder could also be introduced in the fanfiction, adding a new dynamic to the story.
One idea could be that Ladybug and Chat Noir are sent to space on a mission to stop an alien villain who is stealing energy from planets. They have to adapt to zero - gravity combat and use their powers in new ways. For example, Ladybug's yo - yo could be used to grapple onto spaceships.