For a basketball fanfiction, perhaps there is a star player from another school. This player is arrogant and believes he is the best. When Danny starts showing off his skills, this player becomes determined to defeat Danny at all costs and uncovers his secret identity.
In a football - related fanfiction, it could be Dash Baxter. Dash is always competitive and might see Danny's sudden improvement in the sport as a threat. He could try to expose Danny's secrets to get him off the team.
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.
One plot idea could be that Danny's guitar playing attracts a special type of ghost that is drawn to music. This ghost then challenges Danny to a musical duel, and if Danny loses, the ghost will take over the town's music scene. Danny has to use his unique guitar skills and ghost powers to win.
The main villain could be an older and more powerful version of Vlad. Maybe he has finally perfected his cloning techniques and created an army of half - ghost clones. He could be more devious and have a grand plan to take over both the human and ghost worlds.
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 powerful warlock who wants to harness the powers of both Danny and Carrie for his own evil purposes. He could be an ancient being who has been waiting for the right time to strike when both Danny and Carrie are at their strongest.
Danny's parents could be main characters too. They might be trying to throw Danny a normal human birthday party, completely unaware of the ghostly happenings that often surround their son. Their attempts to have a 'normal' celebration could be constantly interrupted by Danny having to slip away to deal with ghost problems. This would create an interesting dynamic between Danny trying to keep his double life a secret from his parents while still enjoying his birthday with them.
Vlad Masters could easily be the main villain. Since he already has his own ambitions and a certain amount of power, in a royal - themed fanfiction, he could be trying to take over the ghostly royal throne. He might see Danny as a threat to his plans, especially if Danny has some newfound royal status, and thus plot against 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, the main character would be Danny Phantom himself, but in two different forms or versions. One would be the original Danny Phantom from the canon source, and the other would be a version created by the fanfiction. There might also be his friends like Sam and Tucker who could play important roles in helping the two Dannys understand each other and deal with any issues that arise from their meeting.