One possible plot could be that Ladybug and Chat Noir are somehow transported to the Dark Souls universe. They find themselves in a desolate and dangerous land, facing powerful enemies they've never encountered before. Ladybug's Lucky Charm ability might work in strange ways, like creating items that have a different use in the Dark Souls world. Chat Noir could use his cat - like agility to navigate the treacherous terrains. Together, they have to find a way back home while also uncovering the secrets of this new and menacing realm.
Perhaps in the crossover, the Miraculous Ladybug world is starting to be overrun by the same kind of dark magic that plagues the Dark Souls universe. Ladybug and Chat Noir start to notice that their powers are being affected. They then enter the Dark Souls world to find the source of this dark magic. Along the way, they meet characters from Dark Souls who either help or hinder them. They have to adapt their fighting styles and strategies to deal with the new challenges and use the unique features of both worlds to their advantage.
A plot could involve the Dark Souls' bosses being influenced by Hawkmoth's akumas. The heroes of Miraculous Ladybug have to fight these akumatized bosses. For example, the Gaping Dragon could be akumatized and become even more powerful and unpredictable. Ladybug and Chat Noir would need to figure out how to break the akuma's hold on these Dark Souls creatures. They might also team up with some of the friendly npcs in the Dark Souls world who recognize their power and the threat of the akumatized bosses.
Ladybug would be very interesting. Her positive and creative approach in Miraculous Ladybug would contrast greatly with the dark and gloomy world of Dark Souls. She could bring a new perspective to the problems and challenges in the Dark Souls world. Her ability to think on her feet and come up with solutions using her Lucky Charm would be fascinating to explore in a world full of death and despair.
Another plot could involve Anna getting a Miraculous. She could be a new hero in Paris, teaming up with Ladybug and Chat Noir. They might face a villain who has the ability to create eternal winter like in Frozen, and Anna's experience with dealing with Elsa's winter could come in handy. Also, the cultural differences between Arendelle and Paris could add some interesting twists to the story.
One idea could be that the Dark Owl discovers a new power within his Miraculous that he has to learn to control. Maybe it makes him see things from the past related to the Miraculous that could change the way he fights villains.
One possible plot could be that Ladybug and Chat Noir are transported to the world of Frozen. There, they meet Elsa and Anna. Maybe they team up to fight a new villain that is causing chaos in both the Miraculous and Frozen universes. For example, a villain might be using the power of the Miraculous to freeze parts of Paris and also trying to take over Arendelle.
One idea could be a crossover with the DC universe. Ladybug and Cat Noir could team up with Superman to stop an akumatized Lex Luthor who has somehow gotten his hands on a Miraculous. Another plot could involve a crossover with the Harry Potter world. Maybe Hawk Moth tries to steal magic from Hogwarts, and Ladybug and Cat Noir have to work with Harry and his friends to stop him.
One interesting story could be about a darker origin for the Miraculous. For example, the Miraculous were originally created for a more sinister purpose but over time were repurposed for good. In this fanfiction, Ladybug and Chat Noir might discover this dark past and have to deal with the moral implications.
One great Miraculous Ladybug crossover fanfiction is the one where the characters cross over with the DC universe. It adds an interesting dynamic as Ladybug and Cat Noir interact with superheroes like Superman and Batman. Another cool one is the crossover with the Sailor Moon world. The magical girl elements blend well together.
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.
One idea could be that the characters from RWBY are transported into the Dark Souls universe. They have to adapt to the harsh and unforgiving world, facing the difficult enemies and complex level designs. For example, Ruby could use her scythe skills in the Dark Souls combat system, but find that the enemies are much more resilient than the Grimm she's used to.
A great plot could involve a portal opening between the two worlds. Harry, Hermione, and Ron accidentally stumble through it. In the Dark Souls world, they find that their magic works differently. They meet characters from Dark Souls who are initially hostile but then realize they have a common enemy in the form of Voldemort's forces that are starting to corrupt the Dark Souls world as well. Harry has to learn the ways of the Dark Souls combat and use his wits and magic to form an alliance. They then return to Hogwarts with new powers and knowledge to face Voldemort in a final battle that combines the magic of Harry Potter and the dark and gritty elements of Dark Souls.