Skulker could also be a great antagonist. He might view the twin brother as a new and exciting target to hunt. Since the twin is likely to be inexperienced in dealing with ghosts at first, Skulker could pose a real threat, and his attacks could force the twin and Danny to work together to defeat him.
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.
A new ghostly entity created specifically for the fanfiction could be an antagonist. This entity could be jealous of the bond between Danny and his twin brother. It could try to drive a wedge between them by causing misunderstandings and creating dangerous situations that would test their relationship.
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.
One possible main antagonist could be a powerful ghost who sees the combination of Danny and his twin sister's powers as a threat. This ghost could be trying to split them up or turn them against each other so that they can't use their powers together to stop his evil plans. Maybe he has some connection to their family's past in the Ghost Zone that makes him particularly interested in them.
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.
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.
Another plot could be that the twin brother is from an alternate dimension where everything is reversed. In this dimension, Danny is the villain and the twin is the hero. When the twin accidentally crosses over to Danny's dimension, he has to figure out how to deal with this 'evil' version of himself while also trying to find a way back home. This would offer a lot of opportunities for character development and interesting power battles.
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.
One possible antagonist could be Vlad Masters. He might want to keep Danny's body for his own evil purposes, like using it to gain more power or to take over Danny's life. Another could be a powerful and vengeful ghost who has a grudge against Danny and sees his body as a way to get back at him.