Perhaps Jack Frost time travels to a parallel universe where his family has a completely different life. His family could be powerful rulers in this universe, and he has to learn to fit in while also trying to figure out how to get back to his own world. Along the way, he forms new relationships with his alternate - family members and discovers new aspects of his own identity.
Older family members, like grandparents, might have some knowledge about time travel in the family's past. They could either be very helpful, guiding Jack through his experiences, or they could be hesitant because they know of the potential risks. Maybe they had a bad experience with time travel in their youth and don't want Jack to go through the same.
One plot idea could be that Jack Frost is drawn to Elsa's ice powers. He visits Arendelle out of curiosity. They could start by being a bit wary of each other but then realize they can learn a great deal from one another about controlling and using their powers in new ways. For example, Jack might show Elsa how to have more fun with her ice, like creating ice sculptures that can move on their own.
A really interesting plot could be a time - travel one. Maybe Female Hiccup travels to Jack Frost's past or vice versa, and they have to adjust to the different time periods while also falling in love. Or perhaps there's a plot where they are both tasked with protecting a special, magical artifact that has the power to change the balance of their worlds, and they have to overcome their differences to succeed.
One idea could be that Elsa and Jack Frost discover they both have the power to control elements in a unique way. They could team up to save a magical world that is being threatened by a dark force that can nullify their powers. Another plot could be that they meet and initially clash because of their different approaches to using their powers, but then gradually learn from each other and fall in love.
One idea could be that Jack Frost discovers a new hidden power within himself while interacting with the Guardians. This power could be related to an ancient magic that the Guardians didn't know about. Another plot could involve Jack Frost having to prove himself to a skeptical Guardian who doesn't believe he's worthy of being part of the group. And a third plot idea might be that Jack Frost and the Guardians team up to face a new and powerful enemy that threatens the balance of the world they protect.
A plot could revolve around Elsa and Jack Frost having to deal with the unexpected pregnancy during a time of unrest in their respective kingdoms. Maybe there are enemies who see the baby as a threat and try to harm Elsa. Jack has to use his powers to protect her and their unborn child. They could also encounter a wise old sorcerer who gives them advice on how to control and enhance the baby's powers even before it is born. As the pregnancy progresses, Elsa learns more about her connection to Jack's magic through their unborn child.
Maybe Jack Frost loses his memory and ends up in the Fairy Tail guild. The members try to figure out who he is and where he came from while he starts to form new friendships and discovers new aspects of his powers. This could lead to a lot of humorous and heartwarming moments as he adjusts to the guild's chaotic but loving environment.
Jack could travel to a time when the Autobots first arrived on Earth and help them understand human culture faster, thus forging stronger alliances earlier. This would change the whole dynamic of the war on Earth.
One plot idea could be that Pitch Black tries to use his powers of darkness to tempt Jack Frost into joining his side, but Jack resists. Their interactions could be filled with tension and drama. For example, Pitch might offer Jack power over winter in a different, more menacing way. Another idea is that they could be forced to work together to defeat a common enemy, and during this time, they start to understand each other better. This could lead to a slow - burn friendship or something more.
One plot idea could be that Jack Frost, being the spirit of winter, accidentally freezes part of the Moon Daughter's celestial realm. They then have to work together to reverse the damage. Jack uses his powers over ice and snow, while the Moon Daughter uses her lunar magic. Another could be a story where they meet during a rare celestial event and have to team up to protect the Earth from an otherworldly threat.