Perhaps an ancient ghost that has been trapped in a time loop for ages. When Danny becomes the new time ghost, it disrupts the loop, and this ancient ghost becomes angry and tries to destroy Danny to restore the loop. The ancient ghost could have some unique powers related to time that make it a formidable foe for Danny.
There could be a group of ghosts who are against any form of change in the ghost world. They see Danny's transformation into a new time ghost as a dangerous deviation from the norm. So, they band together to try and eliminate Danny and reverse his transformation. Each member of the group could have different abilities that they combine to create various challenges for Danny.
It could be another powerful time - manipulating ghost. This ghost might see Danny as a threat to his own plans of ruling over all time periods. He would try to stop Danny from fully understanding and using his new powers.
It could be Vlad Masters. Since he's always been interested in Danny's half - ghost state, he might see Sam becoming a half - ghost as an opportunity to gain more power or to use her against Danny. He could try to manipulate her new powers for his own evil plans.
It could be Sam and Tucker. Since Danny has become a villain, his former friends would likely try to stop him. They would be the main antagonists from Danny's point of view now. They would use their knowledge of Danny and their own skills to try and bring him back to the good side.
Maybe Danny discovers a new type of time - related ghost power. He could start using this power to travel through time and change certain events in the past that were related to ghosts. For example, he might prevent some ghosts from being created or stop a major ghost - related disaster that happened long ago.
The Guys in White could be the main antagonists. If Danny becomes evil, they might see this as their chance to finally capture and experiment on him. They would be constantly chasing him, which would make Danny see them as the main obstacle in his new evil plans, and thus they would be the antagonists in the story.
It could be an evil spirit that wants to use Naruto's new ghostly form for its own dark purposes. Maybe it tries to control Naruto and make him do evil things.
Maybe it's a corrupted version of one of Danny's friends. The friend could have been possessed or influenced by some dark power, and then turns against Danny, causing him so much emotional pain that his heart becomes cold.
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.
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.
It could be Pitch Black. Since Danny, as Jack Frost, is new to the world of the Guardians, Pitch might see him as an easy target to disrupt the balance. He could try to turn Danny back into his Danny Phantom self or just use his in - between state to cause chaos among the winter spirits and the people of Amity Park.