Well, the plot could start with a scientific experiment gone wrong in both their universes simultaneously. This creates a rift that pulls Danny into the Flash's world. There, they find out that a common enemy is trying to use the chaos to gain power. This enemy is using a device that can steal powers. Danny and the Flash team up. Danny's ability to go invisible and intangible helps them in stealth missions, while the Flash's super speed allows them to quickly reach different locations. In the end, they manage to defeat the enemy and Danny finds a way back to his own universe.
If Danny Phantom is adopted by Flash in fanfiction, it could completely change Danny's life. He would be exposed to a whole new set of allies and enemies. For example, he might have to face off against Flash's villains, like Reverse - Flash. At the same time, he could also form new friendships with Flash's friends, like Iris. Danny's powers, which are more supernatural in nature, could be a source of fascination and also a bit of confusion for the Flash and his team. And Danny would have to adapt to living in a world where speed is the most prominent power, which is very different from his own world where his ghost powers set him apart.
I think Eobard Thawne, the Reverse - Flash, could be really interesting. He could use his speed to try and steal Danny's ghost powers. His devious nature and time - traveling abilities would make him a formidable foe. He might see Danny's powers as a way to enhance his own and become even more powerful. And Danny would have to use his unique ghostly skills, like his ability to phase through objects and fly, to outwit the Reverse - Flash.
One possible plot could be that Danny and Ben meet when a powerful interdimensional villain is causing chaos in both their universes. They initially misunderstand each other and get into a small fight. But then they realize they need to work together. Danny could use his ghost powers to phase through objects and surprise the enemy, while Ben could use his various alien forms to combat the villain's different attacks. For example, using Heatblast to counter a cold - based attack from the villain.
Maybe in this crossover, some Pokemon are accidentally sent to Danny's world. Danny has to protect the Pokemon from being captured by the Guys in White while also trying to find a way to send them back. Meanwhile, a certain Pokemon, like Gengar, forms a special bond with Danny because of their shared ghostly nature.
The plot could center around power struggles. The Ghost King might see Danny as a threat to his rule because Danny has such unique powers for a half - ghost. So, he tries to eliminate Danny. Danny, on the other hand, just wants to protect his family and friends. This leads to a series of chases and battles throughout the Ghost Zone and the human world.
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.
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.
Well, imagine if there was a device in the Danny Phantom universe that malfunctioned and created a portal to the Harry Potter world. Danny ends up at Hogwarts during the Triwizard Tournament. He gets involved in the tasks because his powers are seen as an advantage. He might team up with Harry and use his intangibility to pass through obstacles in the maze during the third task. At the same time, he could teach Harry some of his skills, like how to quickly phase through walls to avoid danger.
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.
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.