One interesting crossover could be that Ladybug and Spiderman team up to fight a common villain. Maybe the villain has the ability to create chaos in both Paris (where Miraculous Ladybug is set) and New York (Spiderman's territory). They could combine their unique powers. Ladybug's Lucky Charm and Spiderman's webs could be used in creative ways to outwit the enemy.
Another idea is that there is a magical artifact in New York that is causing problems similar to the Akumas in Paris. Ladybug and Spiderman are both drawn to it. Ladybug's Miraculous Cure might not be enough to fix the damage caused by this artifact, so she has to work with Spiderman to find a more permanent solution. They could also have some cultural exchanges during their adventure, like Ladybug showing Spiderman some French pastries and Spiderman introducing her to some New York pizza.
One interesting crossover could be that the villains from both universes team up. For example, Green Goblin from Spiderman 3 and Hawk Moth from Miraculous Ladybug might join forces. They could create chaos in both New York (from Spiderman) and Paris (from Miraculous Ladybug). This would force Spiderman and Ladybug to work together, which would be a really cool dynamic as they have different powers and fighting styles.
A crossover with the world of Sailor Moon would also be cool. The Miraculous holders could meet the Sailor Scouts. They could exchange their powers and fighting techniques. For example, Ladybug could show Sailor Moon how to use her yoyo in a more strategic way, and Sailor Moon could teach Ladybug about the power of love and friendship in her battles.
One interesting crossover could be a Creepypasta villain somehow getting their hands on a Miraculous. For example, a Creepypasta character like Jeff the Killer might try to use the Cat Miraculous' power of destruction in a really chaotic way. It would create a whole new dynamic as the Miraculous Ladybug heroes would have to face a much more sinister and unpredictable enemy.
One interesting crossover could be the Ladybug and Cat Noir characters getting Autobot or Decepticon powers. Maybe Ladybug could use her Miraculous powers in combination with a Transformer's ability to change forms for some super - cool battles.
A cool crossover in the fanfiction could be a situation where the two universes are in danger simultaneously. Iron Man and Ladybug have to team up to fight villains from both their worlds. Ladybug's Lucky Charm ability could be really useful in unexpected ways when combined with Iron Man's strategic thinking. They might also exchange their knowledge about their respective technologies and magic. Iron Man could be fascinated by the Miraculous and try to understand its science, while Ladybug could learn some new ways of using her powers from Iron Man's engineering skills.
One interesting crossover could be the characters from both universes teaming up to fight a common enemy. For example, Spider - Man's web - slinging skills combined with Ladybug's Lucky Charm power could make for some really cool fight scenes. They could be facing a villain who has the ability to create chaos in both New York (Spider - Man's city) and Paris (where Miraculous Ladybug is set).
Ladybug and Frisk would be really interesting. Ladybug is a strong superhero, and Frisk is a determined human in the Undertale world. Their meeting could lead to some great character development as they both have a strong sense of justice. Ladybug could learn about Frisk's pacifist ways of dealing with enemies, and Frisk could be inspired by Ladybug's courage.
There are some really interesting crossovers. For example, you might find ones where Ladybug's powers are compared to magic in the Harry Potter world. Some fanfictions could have Marinette (Ladybug) suddenly finding herself at Hogwarts and having to navigate both her superhero duties and the magical world. It's a fun concept as it combines the magic and mystery of Harry Potter with the action - packed superhero world of Miraculous Ladybug.
Definitely Superman if it's a DC crossover. His super strength and ability to fly could add a whole new dynamic to the battles against Hawk Moth. He could also be a mentor figure to Ladybug and Chat Noir, teaching them about using their powers more responsibly on a global scale.
Ladybug's power of creation with the Lucky Charm could be enhanced by Spiderman's scientific knowledge. He might be able to analyze the Lucky Charm and figure out new ways to use it. For example, if the Lucky Charm is a simple object like a key, Spiderman could use his knowledge to turn it into a tool to unlock a special device that helps them in their fight.