A powerful ice - demon could be a great antagonist. It could be jealous of Jack Frost's power over winter and Danny's ghost powers, so it tries to take over their powers and rule both their worlds.
It could be Pitch Black. Since Danny, as Jack Frost, is new to the world of the Guardians, Pitch might see him as an easy target to disrupt the balance. He could try to turn Danny back into his Danny Phantom self or just use his in - between state to cause chaos among the winter spirits and the people of Amity Park.
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.
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.
There could be an interdimensional being that thrives on chaos. It sees the meeting of Jack Frost and Sonic as an opportunity to cause mayhem in both of their worlds. It starts by creating disasters in both the wintry realm of Jack Frost and the colorful world of Sonic. To stop it, Sonic and Jack Frost must figure out the being's weakness while also trying to protect their respective homes and friends.
Perhaps a corrupted version of one of their friends or allies. For example, a possessed Tucker or Beast Boy. This would create an interesting dynamic as Danny and Raven would be hesitant to fight their friend at first but would have to find a way to save them from the corruption while also stopping the harm they are causing.
There are indeed fanfictions with Danny Phantom and Jack Frost. Fanfiction writers enjoy taking characters from different universes and melding them together. In the case of Danny Phantom and Jack Frost, it can lead to really interesting stories. One might imagine a scenario where the two discover a hidden connection between the Ghost Zone (from Danny Phantom) and the world of winter spirits (from Jack Frost). Writers can play with their powers, personalities, and backstories to create engaging and original fan - made tales.
Another theme is adventure. They are both from different fantastical worlds, so fanfiction might take them on new quests or battles. For example, they could be on a mission to save a magical artifact that is important to both of their worlds. This can lead to a lot of exciting action scenes and character development as they face challenges together.
Yes, there are likely fanfictions out there with such a premise. Fanfiction writers often come up with creative and unexpected relationships between different fictional characters. They might explore how Danny Phantom's powers and his character would be affected by being Jack Frost's son, and could create really interesting storylines that combine the elements from both universes.
Yes, there are quite a few good ones. You can start by looking on popular fanfiction websites like Archive of Our Own. Some stories might focus on their adventures together, combining the unique powers and worlds of Danny Phantom and Jack Frost.
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.