Another idea is a crossover with a superhero universe. Imagine Danny Phantom's Ghostly Wail being used in a fight against some supervillains. He could cross paths with a young superhero who is initially scared of Danny because of his ghostly appearance. But when they see how Danny uses his powers, especially the Ghostly Wail, for good, they team up. Danny could teach the superhero about the ghostly realm and how to deal with supernatural threats.
Focus on character development. For instance, if you're crossing over with a character from another series, show how Danny's Ghostly Wail affects that character's view of the supernatural. Maybe they were skeptical before, but after seeing the power of the Ghostly Wail, they start to believe. This can create an interesting dynamic between the characters.
One interesting Danny Phantom Ghostly Wail fanfiction could be about Danny exploring the full extent of his Ghostly Wail power in a new dimension. Maybe he discovers that in this dimension, the wail can not only repel ghosts but also heal certain spectral entities.
Jazz, Danny's sister, is also a common character. She might be the one who tries to understand Danny on a more emotional level when the power of the Ghostly Wail starts to take a toll on his mental state. For example, she could be the one to talk him down when he's frustrated about not being able to control it properly.
Yes, there are some interesting ones out there. You can start by looking on popular fanfiction websites like FanFiction.net or Archive of Our Own. Just search for 'danny phantom ghostly harem' and you'll find a variety of stories with different takes on the concept.
One common theme is the supernatural aspect. Since it involves Danny Phantom and ghosts, there's often a lot of exploration of the otherworldly powers and how they interact with the harem dynamic. Another theme could be love triangles or more complex relationship webs within the harem.
For the crossover, a good plot could be that a new type of ghost appears that is immune to Danny's ghostly powers. The Ghost Hunt team, which has encountered a similar situation before, offers their expertise. They introduce Danny to a special type of ectoplasm - based weapon that can harm this new ghost. Danny then teaches them some of his combat moves as a half - ghost to better fight the ghost.
One idea could be that the Totally Spies are sent on a mission to investigate some strange paranormal activities. They then discover Danny Phantom's world and team up with him to stop an evil force that is using both magic and technology to take over the world. Danny could teach them about the Ghost Zone and they could share their spy skills with Danny.
You can try Archive of Our Own (AO3). It has a large collection of fanfictions, and there are likely to be some Danny Phantom Ghostly Lemon ones there.
One popular Danny Phantom Ghostly Lemon fanfiction could be 'The Haunted Encounter'. It has an interesting plot where Danny faces a new type of ghostly threat and his lemon - like powers play a crucial role in the story.