Maybe a mad scientist who has found a way to combine ghostly energy with his own technology. He targets Danny to use him as a power source for his evil experiments. Batman gets involved when he notices the strange energy signatures in Gotham. The scientist would be a formidable antagonist as he has both scientific knowledge and the power of the ghosts on his side.
Another likely antagonist could be the government or some secret agency. They might see Danny as a threat because of his powers. In an angry fanfiction, they could be constantly trying to capture or study him, which would fuel Danny's anger. And Danny would have to fight against them to protect himself and his loved ones.
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.
Well, maybe Danny Phantom gets into some serious trouble in the Ghost Zone. Batman, being the vigilant hero he is, somehow discovers Danny's plight. He could use his high - tech gadgets to enter the Ghost Zone and rescue Danny. Then they might team up to fight against a common enemy that threatens both the human world and the Ghost Zone.
Obviously, the main character would be Danny Phantom himself, but in two different forms or versions. One would be the original Danny Phantom from the canon source, and the other would be a version created by the fanfiction. There might also be his friends like Sam and Tucker who could play important roles in helping the two Dannys understand each other and deal with any issues that arise from their meeting.
Perhaps some rogue ninja from other villages. They might see Naruto as a symbol of Konoha's power and want to take him down to weaken Konoha. These rogue ninjas could be led by a powerful and vengeful shinobi who has a grudge against Konoha. So when Naruto is alone on a mission or something, they attack, and that's when Minato steps in to save him.
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.
Vlad Masters could be a main antagonist. He might be jealous of Danny becoming the Ghost King and try to usurp the throne. Since he has his own ghost powers and a long - standing rivalry with Danny, he would be a formidable foe.
The Joker would definitely be a major antagonist. His chaotic and unpredictable nature would pose a huge threat to Danny. He'd likely be intrigued by Danny's powers and try to use them for his own amusement or evil schemes. Another one could be the Penguin. He might see Danny as a new obstacle in his plans within Arkham or as something he could sell to the highest bidder, perhaps to other villains outside the asylum who are interested in Danny's unique abilities.
The Guys in White could be antagonists. They might see Star's magical powers as a threat and try to capture her for study, not realizing that she's with Danny. This would put Danny in a tough position as he has to protect Star from them while also keeping his own identity a secret.
In this fanfiction, the Observants could be the main antagonists. They are used to being the ones in control and guiding the events in the Ghost Zone. Danny becoming a god would disrupt their carefully crafted order. They would try to undermine his power and influence, perhaps by spreading rumors among the other ghosts that Danny is going to bring chaos. Also, certain groups of rogue ghosts who enjoy causing mayhem in both the human and ghost worlds would oppose Danny. They would be afraid that he will stop their fun and impose strict rules with his new godly powers.