In a Danny Phantom crossover harem fanfiction, Danny could be part of a team - up with other characters from different shows. The girls on the team start to develop feelings for him over time. For example, there could be a magic - using girl who is impressed by Danny's ability to phase through walls and use his ghostly wail. A tech - savvy girl might admire his ecto - energy manipulation. As the story progresses, Danny has to balance his duties as a hero and his relationships with these girls.
Maybe Danny gets transported to a different dimension where there is a harem - like society. The local girls there are intrigued by his ghost powers and his heroic nature. They start following him around, and he has to figure out how to deal with their attention while also trying to find a way back home. Some of the girls might be from different species, which could add more interesting elements to the story, like different cultural norms regarding relationships.
Another plot idea is that the OC is from a different dimension and is brought to the Danny Phantom universe by accident. Their strange and alluring appearance and behavior draw in different characters. The OC might initially struggle to fit in but gradually forms relationships with them, which could turn into a harem situation as they each have their own reasons for being attracted to the OC, like the OC's knowledge of different technologies or magic from their home dimension.
One idea could be that Danny Phantom, while dealing with his normal ghost - fighting duties, attracts a succubus who then creates a harem of various characters from his world. Maybe they all have to work together to fight off a greater evil that is attracted to the combined energy of the harem.
One idea could be that Danny's ghost powers start to malfunction in the TMNT's dimension, causing strange ghostly events that the turtles have to help him fix. Another could be a villain from Danny's world teams up with one of the TMNT's foes, forcing the two groups to work together. Maybe Danny accidentally gets sent to the sewers where the turtles live and they initially mistake him for a new type of mutant.
A possible plot is that a powerful ghost in Danny's world has a connection to an ancient demon in Inuyasha's world. This connection is causing chaos in both worlds. Danny and Inuyasha meet and initially are at odds because they don't trust each other. But as they start to learn about the threat they face together, they begin to work together. Danny uses his energy blasts and Inuyasha uses his Tessaiga to fight the combined forces of the ghost - demon entity.
Well, there could be a story where Danny's ghost sense starts acting up in the Naruto world. It turns out that there are some spirits there that are similar to the ghosts in his world. Naruto thinks Danny is crazy at first but then they discover that these spirits are tied to an ancient curse. Together, they have to find a way to break the curse. Danny uses his ability to communicate with ghosts to learn more about the spirits, and Naruto uses his determination and strength to face any physical challenges that come their way.
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.
One idea could be that Danny and Kim have to team up to stop a new villain who has the ability to manipulate both the ghost world and the regular world. This villain could be causing chaos in both dimensions, and only by combining their unique skills can they hope to stop it.
One idea could be that Danny Fenton and the Mystery Twins, Dipper and Mabel, cross paths while investigating a supernatural mystery that involves both the Ghost Zone from Danny Phantom and the strange anomalies in Gravity Falls. Maybe a powerful ghost has escaped into Gravity Falls and is causing chaos there, and Danny has to team up with the twins to stop it.
One idea could be Danny Phantom teaming up with characters from other Nickelodeon shows to fight a common supernatural threat. For example, he could join forces with the Teenage Mutant Ninja Turtles. They could use their unique skills, Danny with his ghost powers and the Turtles with their ninja skills, to take on a powerful ghost - mutant hybrid enemy.
One idea could be to explore how Danny's ghost powers interact with the dark and tortured world of 'The Suffering'. Maybe he gets trapped in that nightmarish realm and has to use his unique abilities to survive and find a way out. Another idea is to have some of the tortured souls from 'The Suffering' cross over into Danny's world and how he deals with them while keeping his identity as a half - ghost a secret.