One possible plot could be that there is a rift in the multiverse which causes Danny Phantom to cross paths with Gaster. Gaster's knowledge of the multiverse and Danny's ability to traverse different dimensions in his ghost form could be combined. They start to explore the reasons behind this rift and discover that an evil entity is trying to merge different realities for its own malicious purposes. Danny and Gaster then have to use their unique skills to stop this entity and close the rift to restore the balance of their respective universes.
It could be Vlad Masters from Danny Phantom's world. He might follow Danny through the encounter with Gaster and see new opportunities for power in this strange new world. He could team up with some of Gaster's enemies in the new world to cause trouble for Danny and Gaster.
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.
In a 'Danny Phantom meets Castiel' fanfiction, perhaps Danny is drawn to Castiel's world because of some strange energy fluctuations. Castiel senses Danny's arrival and is intrigued. They start to communicate, with Danny explaining his world of the living and the dead, and Castiel sharing about heaven, hell and the angels. Eventually, they find out that there is a dark force trying to merge their two realms and they must stop it.
Obviously, the main character would be Danny Phantom himself, but in two different forms or versions. One would be the original Danny Phantom from the canon source, and the other would be a version created by the fanfiction. There might also be his friends like Sam and Tucker who could play important roles in helping the two Dannys understand each other and deal with any issues that arise from their meeting.
One possible plot is that Jack Frost accidentally enters the world of Danny Phantom. Danny at first is suspicious of him but then realizes they have a lot in common. They start exploring each other's powers and end up having to stop a powerful entity from stealing their powers. It could involve a lot of action scenes where they learn to work together and use their unique skills to their advantage. For example, Danny could use his intangibility to dodge attacks while Jack freezes their opponents. There could also be some comical moments as they get used to each other's different personalities and ways of doing things.
The plot might involve Danny Phantom being mistaken for a meta - human in the Young Justice world. At first, there could be some distrust from the Young Justice members, but as they get to know him and his powers, they realize he can be a great asset. They could end up on a mission to stop a powerful ghost - like being that has invaded their dimension. Danny would use his knowledge of the ghost zone to help them understand this new threat and together they'd come up with a plan to defeat it.
I think Superman would be the most likely. Superman is known for his kind heart and open - mindedness. He would probably see Danny as a young hero in need of some guidance and friendship. He wouldn't be too quick to judge Danny based on his ghostly appearance or powers.
The plot could change in terms of her love interests. Instead of the typical male - led romantic arcs, there could be new female - centered ones. For example, she might have a different type of relationship with a male character, where she takes the lead more often.
Maybe the main villain is an ancient spirit that was once locked away in a dimension between the two worlds. Danny's accidental crossing of dimensions and Castiel's presence somehow break the seal. This spirit wants to destroy both the world of the living (that Danny protects) and heaven and hell (that Castiel is associated with) in order to create its own chaotic realm.
One possible plot could be that Izuku discovers the existence of Danny Phantom through some kind of interdimensional event. He then tries to learn from Danny's way of handling his powers and his secret identity, applying those lessons to his own journey as a hero in training.