There could be a group of human - turned - ghost hunters who are jealous of Danny's powers and want to prove themselves by capturing the ghost kids. They might be using some new, dangerous technology that Danny has to figure out how to counter. They could also be spreading false rumors about the ghost kids, making it even harder for Danny to protect them.
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 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.
Vlad Masters could be an antagonist. He might see the twin brother as a new opportunity to either gain more power or to get back at Danny. He could try to manipulate the twin, using his lack of knowledge about the situation in Amity Park to his advantage.
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.
There could be a rogue ghostly entity that was once a friend or ally of Danny's family. This entity might feel betrayed by Danny's family for some reason and now wants to take revenge on Danny. Since Danny was raised in the Ghost Zone, he is an easy target for this entity. This antagonist could manipulate the other ghosts against Danny and create all sorts of problems for him as he tries to figure out his place in the Ghost Zone and the human world.
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.
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.
A group of human hunters who have decided to target Danny's ghost side could be the antagonists. They might be trying to capture Danny and use him for their own experiments. Danny, feeling violated and angry, decides to take revenge by using his ghost powers to outwit them at every turn, and expose their cruel intentions to the people of Amity Park.
His friends Sam and Tucker could be his allies. They've always been there for him and would likely support him in his new role as the Ghost King.