Maybe Jack Frost kidnaps a human who has a special connection to the sun. He's envious of the sun's warmth and power, so he takes this human in hopes of using their connection to the sun to his advantage. He holds the human captive in his frozen realm, and the human has to find a way to escape while also trying to understand Jack Frost's motives and change his mind.
One possible plot could be that Elsa and Jack Frost meet by chance in a magical realm that bridges their two worlds. They fall in love quickly. Then Elsa discovers she is pregnant. They have to hide from the forces in their respective worlds that would oppose their union because of the pregnancy and the implications it has on the balance of magic.
One possible plot could be that Jack Frost, perhaps out of loneliness or a misunderstanding, kidnaps Elsa. He takes her to his icy realm. Elsa is initially scared but then realizes Jack has his own insecurities. They start to talk, and Elsa, with her power over ice, shows Jack new ways to use his powers. Eventually, they become friends and Jack realizes his mistake and returns Elsa to Arendelle.
One possible plot could be that Jack Frost, being the spirit of winter himself, is intrigued by Elsa's ice powers. They could start by having a friendly competition to see who can create the most magnificent ice sculptures. Their powers combined might accidentally create a new, magical ice realm that they then have to explore and protect together from an approaching threat, like a dark sorcerer who wants to steal their ice magic.
Well, the plot could start with a magical event that binds Jack Frost and Hiccup together in a way that results in pregnancy. They then embark on a journey to understand what this means for them. Hiccup, with his experience with dragons, might try to find a dragon - related solution to any potential problems during the pregnancy. Jack Frost, on the other hand, could use his powers to create a safe and comfortable environment for the expecting. As the story progresses, they might face opposition from some who don't believe in their relationship or the idea of such an unusual pregnancy, but they stand strong together.
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 a magical anomaly causes the worlds of 'Frozen' and 'Rise of the Guardians' to merge. Elsa and Jack Frost meet and are initially suspicious of each other. But as they work together to figure out how to separate the worlds again, they start to develop feelings. Eventually, they share a kiss as a sign of their newfound love and as a way to use their combined ice powers to fix the anomaly.
Maybe they meet in a magical forest. Jack Frost has been feeling lonely in his icy world and the Tooth Fairy, while on her nightly rounds, stumbles upon him. They start to talk, discover they have more in common than they thought, and end up sharing a kiss.
The plot could involve Jack Frost initially being shocked to find his family. He may have thought he was the only one of his kind. His family might have been looking for him all this time. They could introduce him to the family traditions and ways of using their frost powers. And together they might go on an adventure to save their winter realm from some sort of threat.
In a 'jack frost goes back in time fanfiction', he could go back to a time of great winter festivals in a long - forgotten civilization. There, he gets caught up in the local politics and superstitions. The people might think he's a sign of good or bad luck. He has to navigate through their beliefs and also deal with a mysterious force that is causing the winter to be harsher than it should be. Maybe he'll make new friends and learn some old winter magic that he can bring back to his own time.
In this fanfiction, it might also be about Jack Frost finally reuniting with his sister after a long time. He could be excited to share all his adventures with her. They might spend time reminiscing about their past, and then Jack could teach his sister how to skate on the frozen ponds he creates. There could be some comical moments too, like when his sister tries to make her first snowman and it turns out all wonky.