Well, it could be that there is no traditional antagonist. Sam's betrayal could be the main source of conflict, and she could be seen as the antagonist in Danny's eyes for that time being. But if there has to be another force, it could be the entity that influenced Sam to betray Danny in the first place. For example, a powerful and manipulative ghost that has its own agenda and uses Sam as a pawn.
Another possibility is that Sam herself becomes a sort of antagonist - protagonist hybrid. As she betrays Danny, she starts to realize the error of her ways. But her actions have already set things in motion, and she has to struggle against the consequences of her betrayal. So, in a way, she is both the cause of the problem (the antagonist) and the one trying to fix it (the protagonist), adding depth to the story.
Vlad could be a great antagonist. He might see the baby versions of Danny and Sam as an opportunity to finally get rid of Danny for good. He could try to use their weakened state to his advantage and steal Danny's powers once and for all.
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.
One idea could be that Danny and Sam have kids in the future and their kids inherit Danny's ghost powers. The story could focus on how they learn to control their powers while also dealing with normal kid stuff like school and making friends. It would be interesting to see how Sam, as a mom, guides them through this.
One possible scenario could be that Sam betrays Danny's secret identity to someone he doesn't trust, like a ghost hunter. Maybe she does it out of jealousy if Danny starts getting closer to another character.
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.
Sam is also quite popular. Her goth - like personality and her strong - willed nature make her an interesting character. In fanfiction, she can be portrayed in different ways, like being even more involved in Danny's ghost - fighting activities.
His ghost - hunting parents could potentially betray him. If they found out Danny was half - ghost, they might see him as a threat to humanity rather than their son. They've been so dedicated to hunting ghosts that their views could be clouded, and they might not be able to accept Danny for what he is, leading to a heartbreaking betrayal for Danny.
The two main characters are Danny Phantom and Sam. Danny's transformation into an evil version of himself creates a complex dynamic. Sam, with her unique personality and her relationship with Danny, has to step up. She may be the only one who truly understands Danny enough to be able to combat his evil side. And depending on the fanfiction, other characters like Tucker or Jazz might also make appearances and contribute to the story in different ways.
The main characters would obviously be Danny and Sam. Danny, with his ghost - fighting abilities as Danny Phantom, is a key figure. He can fly, become invisible, and has super strength among other powers. Sam, on the other hand, is Danny's strong - willed and intelligent friend. She is often the voice of reason and also has a deep knowledge of the supernatural which helps in their adventures.
Love and friendship are common themes. Their relationship, whether it's a budding romance or a deep platonic bond, is often explored.