You can start by reading a lot of existing fanfictions to understand the tone and themes. Then, come up with your own unique idea. It could be a new adventure for Danny, or a different take on an existing relationship in the show. Write your story, make sure it has a clear plot, interesting characters, and good grammar. Then, you can submit it to the appropriate archive following their submission guidelines.
There are many. Some might focus on alternate universes where Danny has different powers or relationships. For example, there could be a story where he teams up with unlikely allies from other dimensions to fight a new, powerful ghost. Another could explore his family's reaction if his secret was revealed in a more public and unexpected way.
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.
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 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.
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.
There's 'Danny Phantom and the Avengers Crossover'. This fanfic is amazing as it pits Danny against the likes of Thanos. It shows how Danny's unique abilities can be a game - changer in the Marvel universe. The author does a great job of blending the two universes' lore together in a seamless way.
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.
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.