One idea could be that the characters from Hazbin Hotel end up in Paris where the Miraculous Ladybug story takes place. They could interact with Ladybug and Chat Noir, and maybe some of the chaotic energy from Hazbin Hotel causes trouble for the heroes as they try to keep the city safe. For example, Angel Dust might accidentally cause a distraction during an akuma attack.
Another concept could be that a Miraculous gets lost in Hell and the Miraculous Ladybug characters have to journey to Hell (the world of Hazbin Hotel) to retrieve it. This would force them to interact with the demons and sinners there. Alastor might be intrigued by their presence and try to manipulate the situation for his own amusement, while Charlie and Vaggie could be more helpful in guiding them to find the lost Miraculous.
Perhaps there could be a plot where Hawk Moth tries to use the powers or the souls in Hazbin Hotel to enhance his akumas. The Hazbin Hotel characters, not fully understanding the situation at first, get involved. Charlie could see this as an opportunity to prove that redemption is possible even in the Miraculous Ladybug universe and tries to stop Hawk Moth from using the hotel's residents in his evil plans.
If the fanfiction was illustrated, the artist could choose to blend the two styles in different ways. Maybe the background of Hell would be drawn in the detailed and somewhat grotesque style of Hazbin Hotel, while the Miraculous Ladybug characters would retain their smooth lines and bright colors. When they interact, the art style could create a sense of otherworldliness, like two different worlds colliding visually.
Another option is a crossover with Gravity Falls. Dipper and Mabel could end up in Hell for some wild reason, like following a strange portal. They'd meet the Hazbin Hotel crew and chaos would ensue. Dipper would be all about trying to figure out the logic and secrets of this new place, while Mabel would probably befriend some of the more outlandish demons or sinners. Charlie would see the Pines twins as a chance to prove that redemption is possible even for those from other dimensions.
Another idea is that the Phantom is akumatized. His love for Christine could be twisted into a desire for power and control over Paris. Ladybug and Chat Noir have to face this powerful akumatized Phantom. They might discover that the key to defeating him lies in understanding his tragic past and the love he once had. So, they team up with Christine to try and reach the Phantom's true self hidden beneath the akuma's influence.
One idea could be that Marinette and Adrien are cast in a school play as Romeo and Juliet. Their real - life superhero identities start to blend with the characters they're playing, and they find themselves in situations similar to the original story. For example, there are misunderstandings between them due to the interference of Hawk Moth, just like how the feud between the Montagues and Capulets caused problems for Romeo and Juliet.
One creative idea is a crossover with the Jurassic Park world. Imagine Ladybug and Cat Noir having to fight dinosaurs while still maintaining their secret identities. Another is a crossover with the Alice in Wonderland world. The wacky and strange nature of Wonderland would make for an interesting backdrop for the Miraculous heroes.
One possible title could be 'Nathanael and Juleka: A Miraculous Love'. It's simple and straightforward, clearly indicating the two characters from Miraculous Ladybug and that the story is about their relationship.
One idea could be to write about Michael's backstory before arriving at Hazbin Hotel. Maybe he had a difficult life on Earth that led to his damnation. You could explore his relationships, like if he had a family or friends that he left behind.
One idea could be that Marinette from Miraculous Ladybug discovers she is also a princess in the world of The Princess Diaries. She has to balance her superhero duties with princess training. For example, she might use her Ladybug powers to make her princess lessons more fun or to solve problems in the palace that normal methods can't handle.
A new villain could have a weapon that specifically targets Ladybug's powers. When she tries to use her yo - yo or her powers of creation, it backfires and causes her harm. This would force her to rely on her wits and Chat Noir more than ever. They would need to find a way to neutralize the weapon and heal Ladybug.
One idea could be a story where Mylene uses her acting skills to help Ladybug and Chat Noir in a different way. Maybe she pretends to be a decoy to draw out an akumatized villain. It could show her courage and her ability to think on her feet.