It could be a new ghost that emerges, one that is jealous of the Dark Prince's power and wants to take it for himself. This new ghost could manipulate events behind the scenes to make it seem like the Dark Prince is causing more trouble than he actually is, all while planning his own takeover.
Maybe a human villain who has found a way to control ghosts and wants to use the Dark Prince as a weapon. This human could be a scientist who has been experimenting with the supernatural and sees the Dark Prince as the ultimate tool for achieving his goals of world domination. He would be an interesting antagonist as he has different motives from the typical ghostly villains in the Danny Phantom universe.
Another possibility is a group of rogue ghosts who were once loyal to the Dark Prince but now want to overthrow him. They could form an alliance with Danny, thinking that they can use him to get rid of the Dark Prince and then take over the ghost realm themselves. This group of ghosts would add a layer of complexity as Danny has to decide whether he can trust them or not while dealing with the Dark Prince.
Another antagonist could be a human with a grudge against Danny. This person discovers Danny's 'Prince of Darkness' potential and decides to use it against him. They might team up with some lesser ghosts and use dark magic or technology to try and take Danny down. They could be motivated by jealousy or a desire for power.
The opera house owner could be a main antagonist. He might be more concerned with money and his own reputation than the well - being of the performers. He could try to stop the Phantom's attempts to improve the opera, seeing it as a threat to his control.
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.
Well, you could have a time - travel plot in the 'Danny Phantom: The Dark Prince' fanfiction. Danny travels back in time and accidentally changes something that leads to the creation of a more powerful and evil Dark Prince. Now he has to figure out how to fix the timeline while also dealing with this enhanced threat. Or perhaps a plot where the Dark Prince gains the ability to possess humans, and Danny has to find a way to stop him without harming the innocent people.
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.
Obviously, Danny Phantom and Pariah Dark are the main characters. Danny Phantom is the hero, a half - ghost who protects the town from ghostly threats. Pariah Dark is the powerful and evil antagonist that Danny often has to face.
There could be some disillusioned wizards who have lost faith in the current state of the wizarding world. They might see the Dark Prince as a way to change things, even if his methods are dark. These wizards could be those who have been marginalized or have suffered losses during the war. They believe that the Dark Prince has a vision for a different kind of wizarding society, not realizing the full extent of his dark intentions.
In this fanfiction, the Dark Prince might find allies among those who have been wronged by the Ministry of Magic. If there are wizards who have been unjustly punished or whose families have been persecuted, they could be swayed to join the Dark Prince. There could also be some international dark wizards who see an opportunity to expand their influence in the British wizarding world through an alliance with the Dark Prince. Another possibility is that there are some dark entities from other dimensions or realms that the Dark Prince manages to summon and convince to be his allies.