Maybe Ladybug's Miraculous gets mixed up with an MHA student's Quirk. So the student starts to exhibit powers related to both the Miraculous and their original Quirk. This could lead to some really cool fight scenes where they have to learn to control these new combined powers. They might also face challenges as the other characters in both universes are skeptical about this new power combination.
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.
One idea could be a character going back in time to prevent a major villain's origin. For example, if someone went back to stop All for One from corrupting Tomura Shigaraki as a child. Another plot could involve a hero traveling to the future to see the outcome of their current actions and then coming back to make changes. Maybe a student like Izuku Midoriya goes to a future where the heroes have lost and tries to change the present to avoid that fate.
A cool plot might be that the Hogwarts students attend U.A. High School. Hermione would surely excel in academic subjects there, while Ron could struggle a bit more. And Harry could use his experience with fighting dark magic to help in hero training. They could also introduce magic - based quirks to the MHA world, which would change the dynamic of how heroics are done.
One idea could be that the Avengers are somehow transported to the MHA universe. They have to deal with quirks while also trying to find a way back home. Maybe they team up with the students at UA to fight a common villain who has a power that combines both advanced technology and a powerful quirk.
One idea could be that Naruto characters visit the MHA world and have to adapt to the modern society and different power system. For example, Naruto might be intrigued by how quirks work compared to chakra. Another plot could involve villains from both universes teaming up, forcing the heroes from Naruto and MHA to collaborate. They would need to combine their unique fighting styles and powers to defeat the combined threat.
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 a crossover where the Miraculous heroes and Ben 10 team up to fight a common enemy. Maybe an alien villain that is immune to the powers of the Miraculous but Ben's alien forms can counter. Another could be a situation where Ladybug's Lucky Charm combines with one of Ben's alien gadgets to create a super - weapon. And a third could be a story where Ben gets akumatized and the Miraculous heroes have to figure out how to save him without using brute force as his alien forms are very powerful.
One idea could be Multimouse getting lost in an alternate dimension where the rules of the Miraculous are different. She has to figure out how to use her powers in new ways to get back home.
One idea could be that the Miraculous Phoenix has the power to heal not only physical wounds but also emotional traumas. The story could follow a character who is deeply scarred by past events and is chosen by the Phoenix Miraculous. As they learn to use its powers, they gradually heal themselves and those around them.