Vlad Masters could be a likely antagonist. He might see Kim as a threat to his plans regarding Danny. Since he's always looking for ways to get Danny on his side or out of the way, he could try to use Kim as a pawn. Maybe he tries to manipulate Kim into thinking Danny is the bad guy, or he uses his ghost powers and wealth to create problems for both of them.
Well, Danny Phantom has the ability to phase through objects and fly at high speeds, which is really cool. Ben 10 can transform into so many different aliens, each with their own unique powers. And Kim Possible is super athletic and has all those useful gadgets. However, I think Ben 10 might be the most powerful in a crossover. His Omnitrix gives him access to a wide variety of powers that can be used in different situations. For example, if they were in a fight underwater, he could transform into an alien that is adapted to aquatic environments. But again, it's all about how the story is written and what challenges they face.
Kim Possible herself. She's a strong, independent character. In a fanfiction with Danny Phantom, she could be the one to teach Danny a thing or two about being a hero without superpowers (at least in the traditional sense). Her determination and resourcefulness would be a great contrast to Danny's more impulsive use of his powers at times. Plus, seeing her react to the supernatural elements of Danny's world would be fascinating.
I think Superman would be the most likely. Superman is known for his kind heart and open - mindedness. He would probably see Danny as a young hero in need of some guidance and friendship. He wouldn't be too quick to judge Danny based on his ghostly appearance or powers.
In a crossover, characters from the other franchise might betray Danny. For example, if it's a crossover with a show about vampires, a vampire character who seems friendly at first could betray Danny for his ghost powers. They might want to use his powers for their own dark purposes. Another likely candidate could be a character who is jealous of Danny's abilities. Maybe in a crossover with a superhero world, a sidekick who feels overshadowed by Danny's powers and popularity might be the one to betray him.
The Joker could be a great villain. Imagine him getting his hands on some ghostly technology or artifacts from Danny's world. He would use it in his own chaotic and unpredictable way, causing mayhem in Gotham and also attracting Danny's attention. Danny and Batman would have to work together to stop the Joker's mad schemes.
Danny and Kim are often paired up. Their different skill sets make for an exciting combination. Danny with his ghost - fighting abilities and Kim with her acrobatic fighting skills and access to various gadgets.
Perhaps a corrupted version of one of their friends or allies. For example, a possessed Tucker or Beast Boy. This would create an interesting dynamic as Danny and Raven would be hesitant to fight their friend at first but would have to find a way to save them from the corruption while also stopping the harm they are causing.
Perhaps an organization made up of both Pokemon world villains and some of Danny's foes. They could be working together to create a portal that would merge the two worlds in a way that benefits them. They would be the main antagonists as they cause all kinds of chaos, stealing both Pokemon and technology from both worlds to achieve their goals.
The main antagonist could be one of the powerful and malevolent entities from 'The Suffering'. For example, the big bad demon or the tortured soul that has been corrupted by the darkness of that world and now wants to spread it to Danny's world.
One idea could be that Danny and Kim have to team up to stop a new villain who has the ability to manipulate both the ghost world and the regular world. This villain could be causing chaos in both dimensions, and only by combining their unique skills can they hope to stop it.