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 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.
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.
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.
The plot could involve some sort of mystery related to the pregnancy. Like, there could be a ghostly force that is either helping or hindering the pregnancy. And they have to figure out what it is and how to deal with it. Also, their friends like Tucker could play important roles in supporting them.
Perhaps in this '10 Years Later' fanfiction, Danny has become a sort of legend in the world. He has to deal with younger generations of ghosts and humans who either idolize him or are skeptical of his methods. There could be a new villain who is aware of Danny's past and tries to use it against him. Danny may also be exploring new aspects of his powers that he didn't know about before, like the ability to travel to different dimensions related to the ghost zone.
Well, it could be about Danny's ghost powers being exposed to the whole town. Maybe someone discovers his secret lair and reveals it to everyone, causing chaos as the townspeople have to deal with the fact that a half - ghost has been living among them.
A plot could be that Dash discovers Danny's secret identity as Danny Phantom. At first, he's shocked and a bit scared. But then, he realizes that Danny has been using his powers for good. He decides to keep the secret and even helps Danny out in some small ways, like covering for him when he has to disappear to fight ghosts. This new - found alliance between them could grow into a real friendship over time.
Well, it could be about Danny having a whole new set of adventures with Amber as his guardian. Maybe Amber has some unique powers or knowledge that helps Danny in ways his normal family couldn't. For example, she could teach him new ways to control his ghost powers that are different from what he learned on his own.
One possible plot could be that Danny's new fangs give him some unexpected powers. Maybe he can now use them to enhance his ghostly attacks, like injecting a special type of ectoplasm into his enemies. Another idea is that the fangs are a sign of a new transformation phase, and he has to learn to control this new aspect of himself while also dealing with his normal life as Danny Fenton.