A powerful, ancient ghost that has been sealed away in this other world for centuries could be the main antagonist. Danny's arrival might accidentally break the seal, and this ghost, with its vast and unknown powers, sees Danny as a threat to its new - found freedom and plans to destroy him. This ghost could have unique abilities that Danny has never faced before, making it a formidable foe.
It could be an alternate version of Vlad Masters. In this other world, he might have achieved total domination over both the human and ghost realms. Danny would then have to face this all - powerful Vlad and find his weaknesses in this new context.
Maybe the main antagonist is a group of rogue ghosts that have formed a sort of anti - human coalition in this other world. They view Danny as a symbol of the human - ghost balance in his own world and want to prevent him from bringing any of that 'order' to their world. They use their combined powers and strategic thinking to constantly thwart Danny's attempts to understand and navigate this new 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.
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.
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.
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.
A dark sorcerer from Claw of the Wild who has learned about the Ghost Zone and wants to use its power for his own evil plans. He sees Danny as a threat because Danny has the ability to cross between worlds and might stop him. So, he tries to capture Danny and use his powers to open a portal to the Ghost Zone and unleash all kinds of dark forces.
Perhaps an organization made up of both Pokemon world villains and some of Danny's foes. They could be working together to create a portal that would merge the two worlds in a way that benefits them. They would be the main antagonists as they cause all kinds of chaos, stealing both Pokemon and technology from both worlds to achieve their goals.
The main antagonist could be a powerful ghost who has a vendetta against Danny. Maybe it's a ghost that Danny defeated before, but who has come back stronger and is now seeking revenge. This ghost could hunt Danny down as he runs away, causing all sorts of trouble for him.
It could be a ghost hunter who is specifically targeting Danny because of his new fangs. This hunter might think that Danny's fangs are a sign of him becoming more dangerous and decides to take him down before he causes any real harm.