One common theme is the struggle between good and evil within Danny. It often shows how his normal self battles his evil side. Another theme is the impact on his relationships. His friends and family have to deal with this new, dangerous Danny.
One interesting Danny Phantom Danny Evil fanfiction could be 'The Dark Turn'. In it, Danny's evil side really takes over and he starts to cause chaos in the town. It shows a different side of his character and how the relationships with his friends and family change as a result.
One idea could be that Danny turns into a girl due to a ghostly spell gone wrong. In the fanfiction, he could struggle with the new body while still having to fight ghosts. Maybe his friends don't recognize him at first, leading to some comical and tense situations.
In Danny Phantom Evil Danny fanfiction, the way Evil Danny interacts with other characters can be really fascinating. He might manipulate his friends or foes in unexpected ways. For example, he could use his powers to deceive them into doing things that serve his evil plans. This creates a lot of tension and interesting plot twists.
Another theme is the struggle between good and evil within the Danny Phantom universe. In these fanfictions, the balance is tipped towards evil, and we see how the other characters deal with that. For example, Sam and Tucker might have to find new ways to combat an evil Danny, which involves a lot of moral and ethical decisions on their part. They have to decide how far they are willing to go to save their friend and protect the town.
Perhaps in this fanfiction, Danny as a kid would have a different perspective on the ghost world. He might be more curious and less jaded. His appearance as a kid could also change how the ghosts interact with him. Some ghosts might underestimate him, which could be an advantage for Danny. He could use this to his benefit when dealing with them, while also trying to figure out how to turn back to his normal self.
Perhaps there could be a love interest that is unique to his female form. Maybe someone who didn't notice Danny before as a boy, but now is attracted to him as a girl. This could add a whole new layer of drama and romance to the fanfiction, as Danny has to figure out how to handle this new relationship while still dealing with his ghost - hunting duties.
Well, one possible plot could be that Danny, while on an underwater mission to stop some ghostly threat near the ocean floor, gets hit by a strange spell or artifact that turns him into a mermaid. He then has to deal with his new form while also trying to keep his ghost - fighting duties going. Maybe he discovers new powers related to his mermaid form that help him in his battles.
I'm not sure of specific titles off the top of my head, but you can start by looking on popular fanfiction websites like Archive of Our Own. Just search for 'Danny Phantom Evil Danny' and you'll likely find a bunch of interesting reads. Check out the ones with high ratings and lots of reviews.