Well, you could start with Danny accidentally opening a portal that connects his Amity Park to the world of 'The Suffering'. As he ventures into this new and terrifying place, he discovers that the rules of his ghost powers are different there. He might have to team up with some of the more 'human - like' entities in 'The Suffering' who are also trying to escape the eternal torment. This would create a really interesting dynamic as Danny tries to balance his own morals and values with the harsh reality of this new world.
For a 'Danny Phantom The Suffering Crossover Fanfiction', consider a plot where the main villain from 'The Suffering' becomes aware of Danny's half - ghost existence and sees it as a way to either gain more power or finally break free from his own suffering. Danny, on the other hand, has to not only face this powerful new enemy but also deal with the psychological impact of being in such a desolate and hopeless world. He could use his wits and the support of his friends (if they somehow end up there too) to try and defeat the villain and close the portal between the two worlds.
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.
One idea could be that Danny's ghost powers somehow interact with the dragons in HTTYD. For example, his intangibility might make him able to pass through dragon fire without getting burned, and this could be a crucial skill when dealing with a new, more dangerous breed of dragons. Another idea is that Danny gets transported to the world of HTTYD and has to use his ghostly abilities to fit in and help the Vikings. Maybe he can use his flight ability to race against the dragons, which would be an exciting plot point.
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.
The main antagonist could be one of the powerful and malevolent entities from 'The Suffering'. For example, the big bad demon or the tortured soul that has been corrupted by the darkness of that world and now wants to spread it to Danny's world.
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.
One possible interesting plotline could be Danny facing a new and more powerful ghost that has a personal vendetta against him. This would test his limits and force him to discover new powers or ways to fight.
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 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 gets an illness that affects his ghost powers. For example, he might start losing control over his intangibility or invisibility. This could lead to some really comical or dangerous situations. Another idea is that his friends try to find a cure for his strange ghost - related illness while also keeping it a secret from the town.
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.