The Fright Knight could also be a major antagonist. He was once a powerful servant of Pariah Dark, the previous Ghost King. He may see Danny as an unworthy successor and try to overthrow him to restore the 'true order' of the Ghost Zone as he sees it.
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.
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.
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.
There could also be some new ghost characters that are introduced as part of Danny's new role. Maybe a powerful ghost who was once an enemy but now has to work with Danny as he's the king. Or a young ghost who looks up to Danny and becomes his sidekick of sorts in the Ghost Zone.
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.
Well, Danny is obviously one of the main characters. He's the King of the Ghost Zone in this fanfiction, so he's at the center of most of the action. Sam and Tucker are likely to be important characters too. They often support Danny in his adventures in the regular Danny Phantom series, and in this fanfiction, they could be helping him rule or deal with problems in the Ghost Zone.
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.
Well, it could be about Danny having to face various challenges from other powerful ghosts who don't want him to be the king. Maybe he has to prove his worth through battles and outsmarting his enemies.
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.