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.
A new villain created specifically for the crossover could be really interesting. This villain could be a scientist who has somehow harnessed the power of both the Miraculous and the Speed Force in a wrong way. He's bent on using this power to reshape the universes according to his own twisted vision. He has the ability to move at high speeds like The Flash and also use some kind of dark magic similar to Hawkmoth's akumas.
Perhaps it could be Hawkmoth working with a speed - based villain from The Flash's world, like Zoom. Hawkmoth wants the Miraculous for his own reasons as usual, and Zoom wants to prove he's the fastest by taking on The Flash and these new heroes. They could team up and create a lot of trouble for Ladybug, Cat Noir and The Flash.
It could be really exciting. Picture Ladybug and Cat Noir suddenly finding themselves in Central City, where The Flash operates. The Flash might be initially confused by their powers but then team up with them to fight a common villain. Maybe a super - powered criminal who has the ability to steal both the Miraculous and the Speed Force. They would combine their unique abilities, like Ladybug's creation power, Cat Noir's agility and The Flash's super speed to take on the bad guy.
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.
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.
Title: 'The Rise of the Shadow Serpent'. In this fanfic, my villain OC is the Shadow Serpent. She was once a kind-hearted girl, but after being betrayed by those she trusted, she turned to the dark side. She uses her power to create illusions and chaos in Paris. Ladybug and Chat Noir face a tough battle as they try to figure out her motives and weaknesses.
The Reverse Flash could also be a main villain in this crossover. He travels to the Smallville universe to cause trouble. He might target Clark, thinking that by eliminating him early, he can change the future in his favor. This would force Barry and Clark to work together to stop the Reverse Flash's evil plans.
A new character could be introduced as the main villain. Maybe a corrupt police officer who turns to the dark side. He could use his knowledge of the law and the city's security systems to commit crimes and frame the Miraculous heroes. He would be a formidable opponent as he has insider knowledge.
An evil alien warlord could be the main villain. This warlord could be bent on conquering Earth and other planets in the universe. He might see the Miraculouses as a threat and try to destroy Ladybug and Chat Noir first.
Hawkmoth is an obvious choice. His whole mission is to obtain the Miraculouses, and in a death fanfiction, he could succeed in a more devious way than in the original series, directly or indirectly causing Ladybug's death.
A possible main villain could be a character from Ladybug's past who has discovered her identity and is using that knowledge to create chaos. They could be blackmailing her or trying to steal her Miraculous by setting up elaborate mysteries and traps for her to solve.
Maybe the Reverse - Flash. He could somehow cross over into the Percy Jackson world. He'd be a difficult adversary for both Percy and Flash. He could use his negative speed force to slow down Percy's reflexes and disrupt Flash's connection to the speed force. Percy would have to rely on his wits and the power of the sea to fight back, while Flash would have to find a way to overcome the Reverse - Flash's power - draining abilities.