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.
Well, it could be that Jack Frost is creating his icy landscapes when Sonic zooms by. Sonic is curious about this new, frosty world and Jack Frost is equally intrigued by Sonic's speed. They might start off a bit wary of each other but then realize they can team up. Maybe there's a common enemy that likes to disrupt both Sonic's green, vibrant world and Jack Frost's wintry domain. So they combine Sonic's speed and Jack Frost's ice powers to defeat this foe.
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.
Thor might be a great ally for Jack Frost. Thor is used to dealing with magic and powerful beings from different realms. He would be less likely to be intimidated by Jack Frost's powers. They could both use their elemental - related powers (Thor with lightning and Jack Frost with ice) to create powerful combined attacks against their enemies. For example, Thor could strike his lightning into Jack Frost's ice structures, creating a powerful blast of energy.
It could be Dr. Robotnik teaming up with the Fratellis. Dr. Robotnik might see the potential in the Goonies' treasure or the area they live in for his own evil plans. He teams up with the Fratellis who are already villains in the Goonies' world. Together, they create chaos and try to stop Sonic and the Goonies from achieving their goals, whether it's protecting the treasure or saving Sonic's friends.
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.
Sonic and Pikachu would be really interesting. Sonic's fast - paced and adventurous nature combined with Pikachu's cute but powerful personality could lead to some great comical and action - packed scenes. For example, Sonic could be running around really fast and Pikachu could be trying to keep up on his little feet while zapping enemies.
The main antagonist could be an evil sorcerer who has corrupted the Black Knight and is using him to carry out his dark plans. This sorcerer could be after Sonic's speed power to enhance his own magic.
His parents could be in his family. Maybe his father is an ancient frost giant with great power over ice and snow, and his mother could be a gentle spirit who gives life to the frozen landscapes. There might also be siblings, like a sister who can create beautiful ice flowers.
It could be Aku. Since he is the main villain in Samurai Jack's world, his power and evil nature would make him a great antagonist. He could see Harry's magic as a threat and try to eliminate him. And if he teams up with some of the dark forces from the Harry Potter universe like Voldemort or the Death Eaters, it would be even more challenging for Harry and Jack.
In this fanfiction, Elsa and Jack Frost are the main characters. Elsa is a complex character, initially feared in her own kingdom because of her powers. Jack Frost, on the other hand, is a more free - spirited character who loves to play with children and spread winter magic. Their meeting in the fanfiction likely brings out new aspects of their personalities as they interact.