One idea could be that Danny's ghost powers are mistaken for some sort of demonic or otherworldly power by the Winchesters. They might initially try to hunt him down, but then realize he's not a threat and they team up to fight a greater evil that is related to both the ghost world and the supernatural creatures they usually deal with.
I like the idea of a Danny Phantom and Ben 10 crossover. Ben has the Omnitrix which gives him access to different alien forms, and Danny has his ghost powers. They could either be rivals at first, each thinking their powers are better, or they could quickly team up to face a threat that requires both their unique abilities. It would be a fun clash of two very different power - based universes.
One idea could be that Danny and the Winchester brothers team up to fight a ghost that is causing chaos in both the Ghost Zone and the human world. Maybe it's a powerful entity that has the ability to cross between dimensions easily.
Castiel and Danny would be an unexpected but cool pairing. Castiel is an angel with his own unique powers and view of the world. Danny's half - ghost nature would be something completely new to Castiel. They could have some really deep and strange conversations about the nature of life, death, and the afterlife. And when it comes to fighting enemies, their combined powers could be really effective.
One idea could be that Danny, as a half - ghost, has to prove himself to the Ghost King. Maybe there's a tournament in the Ghost Zone where Danny enters to show his worth. This could lead to some epic battles and alliances with other ghosts.
One idea could be that Danny, with his ghost powers, ends up in the RWBY universe. He could team up with Team RWBY to fight off Grimm that are being influenced by some sort of ghostly energy. His intangibility and flight would be really useful in battles.
In a crossover, characters from the other franchise might betray Danny. For example, if it's a crossover with a show about vampires, a vampire character who seems friendly at first could betray Danny for his ghost powers. They might want to use his powers for their own dark purposes. Another likely candidate could be a character who is jealous of Danny's abilities. Maybe in a crossover with a superhero world, a sidekick who feels overshadowed by Danny's powers and popularity might be the one to betray him.
Definitely Danny and Dean. Dean is all about hunting monsters and his initial reaction to Danny, a half - ghost, would be very interesting. He might be skeptical at first but then see Danny's potential in fighting the supernatural threats that are a bit out of his usual realm.
I think Danny Phantom crossover fanfictions can be really exciting. They bring together different universes and characters, creating unique and unexpected storylines. For example, a crossover with a superhero universe could lead to Danny teaming up with other heroes to fight a common enemy. It's a great way to explore new character dynamics and expand the Danny Phantom world.
A common theme is the struggle between good and evil within Danny. When he's in his evil form in crossovers, it often shows the fight to bring him back to the good side.