If it's a crossover with the Teen Titans, Robin would be an excellent character to include. He's a great strategist and his combat skills could come in handy. Raven, with her dark powers, could have an interesting dynamic with Hawk Moth. Also, from the Inuyasha world, Inuyasha himself. His swordsmanship and half - demon powers would add a new flavor to the fanfic.
In a crossover with Sailor Moon, Sailor Moon herself would be a great addition. She has her own powers and a strong sense of justice, just like Ladybug. Tuxedo Mask could also be interesting to pair with Cat Noir. Another good character could be Naruto from the Naruto universe. His ninja skills and determination would blend well with the Miraculous Ladybug world.
One possibility could be Reverse - Flash. He could be interested in the Miraculous powers and try to steal them for his own evil purposes. He might see the power of the Miraculous as a way to enhance his own speed or gain new abilities.
Obviously, Ladybug and Chat Noir are the heroes in the Miraculous world, so they would be main characters. In Frozen, Elsa and Anna are the key figures. However, we might also see other characters like Tikki and Plagg from Miraculous having some interactions with the Frozen characters. For example, Tikki might be fascinated by Elsa's ice powers and try to understand how they work compared to the Miraculous powers.
It could include a transformation of the Princess Diaries' setting into a more Miraculous Ladybug - like world. For instance, the kingdom in The Princess Diaries could be under threat from an akuma - like force. And the characters from both universes would have to work together to stop it. Maybe Mia could be given a temporary Miraculous to help in the fight.
Well, obviously Ladybug and Chat Noir should be in it. They could interact with the cousins and help them learn about their powers. Then there could be the villain characters like Hawk Moth. His presence can create conflict and challenges for the cousins. Another good character could be Master Fu. He can guide the cousins in understanding the deeper meaning and responsibilities of the Miraculous.
Obviously Ladybug (Marinette) and Cat Noir (Adrien) are must - includes. Their dynamic during a sleepover would be really interesting, like when they're not in superhero mode but still have that connection. Alya would also be great as she's Marinette's best friend and is always eager to be involved in anything Miraculous - related. And Tikki and Plagg, the kwamis, could add a lot of humor. They could be causing little bits of chaos while the humans are trying to have a normal sleepover, like stealing snacks or making strange noises to get attention.
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.
You could include characters from other crime - themed games like Tommy Vercetti from Grand Theft Auto: Vice City. His experience in the crime world could create some interesting dynamics with the Mafia 3 characters. They could either be rivals or form an uneasy alliance.
Marinette and Adrien are obvious choices. Their relationship, both as normal students and as superheroes, provides a lot of material for fanfiction. Their crushes on each other, and the comical misunderstandings that often occur, can be explored in a school setting.
In a crossover with 'Powerpuff Girls', the Professor could be an interesting character to include. He might study Zim's technology and try to find a way to use it for good, while the Powerpuff Girls are busy fighting Zim's minions. And Zim would be both fascinated and enraged by the super - powered girls.
Definitely Deadpool. His chaotic and fourth - wall - breaking nature would add a whole new level of craziness to the Suicide Squad. He could have some hilarious interactions with Harley Quinn, like competing for the 'craziest character' title. And his regenerative abilities would make him a valuable asset on dangerous missions.