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.
In a 'Danny Phantom becomes villain' fanfiction, it could be that Danny has a major falling out with his friends and family. He feels betrayed and abandoned. This emotional turmoil causes him to snap and turn to the dark side. He then uses his ghost powers to take revenge on those who he thinks have wronged him, becoming a feared villain in the process.
One great 'Danny Phantom' villain fanfiction is 'The Dark Reign of Technus'. It delves deep into how Technus could take over the entire town's technology infrastructure and turn it against everyone. The way the author describes Danny's struggle against this high - tech tyranny is really engaging.
His powers could become more ice - based. Instead of the normal greenish - blue energy, his blasts might turn a frosty white. His ability to fly could also be affected, with a cold trail following him as he moves through the air.
Obviously, Danny Phantom and Dan Phantom are the main characters. Danny is the original form, a heroic character, and Dan is his more evil and powerful transformation.
One possible storyline could be that Danny is pushed to the edge by constant bullying at school and the stress of hiding his ghost powers. This leads him to gradually embrace the darker side of his powers and become Dark Danny. For example, Dash's continuous taunts and the school's overall negative attitude towards him make him lose faith in humanity.
Danny's relationships change drastically. He distances himself from his friends and family. He no longer values their opinions or support. Instead, he might see them as obstacles or even threats. His friendships with Sam and Tucker are strained as he starts to engage in activities that they find morally wrong, like using excessive force on other ghosts or humans who get in his way.
Well, he might bring some unique changes. He could use his ghost powers to protect the kingdom in a very different way. Maybe he'd make new laws that are fairer to both the living and the dead.
Obviously, Danny Phantom is the main character as it's about his transformation into Dan. His friends like Sam and Tucker would also be important characters. They would likely play a role in trying to either stop the transformation or help Danny deal with the consequences. And of course, his ghostly foes might also be involved, as they could either take advantage of his situation or be affected by the power shift if he becomes Dan.
Well, perhaps Danny Phantom, in this fanfiction, could be turned into a mother due to some magical mishap. Maybe he has to deal with the daily tasks of motherhood like taking care of 'phantom kids' who have unique powers. He might struggle to balance his own heroic duties with the newfound maternal responsibilities.