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.
Another ghost, perhaps a very powerful and ancient one from the Ghost Zone. This ghost could have been secretly manipulating Danny all along, waiting for the right moment to turn him evil. Once Danny is evil, this ghost could try to use him as a pawn in a larger plan to take over the human world or the entire Ghost Zone. So this powerful ghost would be the main antagonist in the story.
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 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.
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.
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.
A new and powerful ghost that Danny has never faced before could be the main antagonist. This ghost could have unique powers that Danny isn't prepared for, and in their first encounter, it manages to defeat and kill Danny. This would set the stage for a story of revenge or trying to bring Danny back.
In the 'danny phantom is a lion fanfiction', the main antagonist could be a group of anti - phantom scientists who capture Danny when he's in lion form. They want to study his unique physiology and use his powers for their own evil experiments. They see him as a threat to their control over the human - ghost interactions and will stop at nothing to keep him locked up and experiment on him.
A new, powerful ghost that hates the Christmas cheer could be the main antagonist. This ghost could be spreading gloom and trying to cancel Christmas in Amity Park. Danny would have to figure out the ghost's motives and stop it from ruining everyone's Christmas. For example, it might be a ghost who was once a Scrooge - like figure in life and now wants to make others as miserable as he was.
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.
Well, it would be the evil Danny Phantom. Consider this: he has gone through some transformation that has made him evil. Now he is using plants in ways that are harmful or against the natural order. Sam, who has a connection to plants and a moral compass, is in direct opposition to him. He creates the conflict in the story by his evil deeds, and Sam has to find a way to overcome his actions, so he is the main antagonist.
A powerful ghost that has taken over the Circus Gothica and is using it to draw in innocent souls. This ghost might have a vendetta against Danny specifically or just be generally evil and want to spread chaos.