One possible plot could be that Danny, through some sort of interdimensional portal accident, ends up in the Star Wars Rebels universe. He then uses his ghost powers to help the Rebels fight the Empire. Maybe his intangibility could be used to sneak past Imperial security or his ecto - blasts could be a new weapon against stormtroopers.
Yes, there are quite a few. You can start by looking on fanfic platforms like Archive of Our Own. Some fanfics might combine the unique ghost - hunting elements of Danny Phantom with the epic space battles and Rebel - Empire dynamics of Star Wars Rebels in really creative ways.
First, you need to research both universes well. Know the characters, their abilities, and the settings. For Danny Phantom, understand his ghost powers and the rules of the Ghost Zone. In Star Wars Rebels, be familiar with the different species, ships, and the power of the Force.
One great fanfiction could be a crossover where Danny Phantom's powers are used in the Star Wars Rebels universe. Maybe Danny accidentally gets transported there and has to use his ghostly abilities to help the rebels. For example, his intangibility could be used to infiltrate Imperial bases unnoticed.
One idea could be to have Danny Phantom's powers in the Star Wars universe. For example, he could use his intangibility to sneak past stormtroopers or his ghostly energy blasts as a new type of weapon. Maybe he gets caught in a wormhole and ends up in a galaxy far, far away and has to find his way back home while getting involved in the Galactic Civil War.
Well, perhaps in this fanfic, the Rebels find themselves in a situation where they have to operate in the 'shadow' in a more literal sense. They might be infiltrating a secret base or a shadowy organization. This could lead to all kinds of adventures, like dealing with double agents within the group and uncovering secrets that could change the course of their fight against the Empire. They may also encounter new allies or enemies in this shadowy world.
A 'Star Wars Rebels Epilogue Fanfiction' might tell the story of how the Rebellion's victory is being commemorated. There could be a grand ceremony on Lothal, with all the main characters in attendance. During this, we could learn more about the characters' inner thoughts and feelings. For instance, Ezra might be reflecting on his time away and how much he has missed his friends. Or, it could show the characters adapting to a more peaceful life. Sabine might find it hard to let go of her warrior ways at first, but gradually learns to embrace a new role in the rebuilding of the galaxy.
The plot could involve the Rebels using the knowledge from the Clone Wars to their advantage. Maybe they discover a long - forgotten battle strategy that was used by the Jedi in the Clone Wars. They then adapt it to a mission against the Empire. Also, the characters could have different reactions to the Clone Wars heroes and villains. Hera might admire some of the Clone Wars leaders, while Zeb could be more critical of the actions of some of the clones.
Danny could be a new type of being in the Star Wars universe. He might be seen as a mysterious entity with his ghost - like powers. He could start off as an outsider, like Luke Skywalker was on Tatooine, but with his own set of abilities that make him both feared and respected. For instance, his invisibility could be a huge advantage in espionage - like missions.
In this fanfiction, it could be that Danny Phantom meets a Danny Phantom from a parallel fan - made story. At first, they don't trust each other. But as they learn more about each other's worlds, they realize that there are some common elements that are being threatened. It could be that a powerful ghost is trying to rewrite all the fanfic universes related to Danny Phantom. So, they must combine their strengths and knowledge to stop this from happening.
Perhaps there is a magical artifact in the MLP world that has the power to disrupt the balance between the living and the dead. Danny Phantom senses this disturbance and travels to the MLP world. Applejack, being the responsible pony she is, gets involved. They have to go on a quest to find the artifact and keep it safe. During their journey, they face many challenges, like dealing with other ponies who don't trust Danny at first, and they also have to overcome the obstacles set by the villains who want to use the artifact for their own evil purposes.