One idea could be that Elsa's powers somehow link to a magic in the Once Upon a Time world. Maybe her ice powers are the key to unfreezing a character who has been trapped by a dark curse. For example, she could help break the spell on Emma Swan's parents. Another plot could involve Anna getting lost in the Enchanted Forest and having to team up with characters from Once Upon a Time like Snow White and Prince Charming to find her way back to Arendelle. A third idea is that the Evil Queen from Once Upon a Time tries to steal Elsa's powers to use for her own evil deeds, creating a big conflict between the two universes.
A great plot for Frozen fanfiction could be a time - travel story. For example, Elsa is sent back in time to when her parents were young. She has to figure out how to get back to her own time while also trying not to change the past too much. But she might also learn some secrets about her family's past and her own powers. Another could be a story about a competition in Arendelle. Anna decides to hold a magical - themed competition, and people from all over come to show off their skills. Elsa has to judge, and in the process, she meets someone who seems to have powers similar to hers but uses them for evil. Anna, Elsa and Kristoff then have to stop this person.
One idea could be that Elsa's powers are mistaken for some sort of new super - weapon by the villains in the Avengers universe. So, the Avengers have to figure out if she's a threat or an ally. Another could be that the Avengers get trapped in Arendelle during a mission and have to work with the Frozen characters to get back home. Maybe Thor could teach Kristoff some combat skills while Elsa helps Iron Man with his suit's temperature control system.
One idea could be that Tarzan, while exploring the jungle, stumbles upon a strange icy portal which transports him to the world of Frozen. He meets Elsa and is amazed by her ice powers. Tarzan, with his jungle skills, could help Elsa understand the natural world in a different way. Maybe he shows her how animals communicate and live in harmony in the jungle, which in turn gives Elsa new ideas about how to interact with the spirits in her own world.
One idea could be a crossover with the world of Narnia. Harry and his friends could accidentally stumble into Narnia through a magical portal. They would then have to adapt to the new world, deal with the White Witch, and maybe use their own magic to help the Narnians. Another plot could involve a crossover with the Lord of the Rings. The Hogwarts students could be transported to Middle - earth to help in the fight against Sauron. They could use their unique spells and magical abilities in battles against orcs and other dark forces. A third idea might be a crossover with Alice in Wonderland. Harry could follow a strange magical creature down a rabbit hole and end up in Wonderland, where he has to solve the crazy riddles and deal with the mad characters while also trying to find his way back to Hogwarts.
One interesting idea could be a crossover with "Moana". Elsa's ice powers and Moana's connection to the ocean could create a really cool dynamic. Maybe Elsa accidentally freezes part of Moana's ocean and they have to work together to fix it. Another could be with "Tangled". Rapunzel's long hair and her adventurous spirit combined with Elsa and Anna's journey in Frozen 2 would be exciting. Rapunzel could use her hair to help them climb mountains or cross chasms. A third idea is a crossover with "Beauty and the Beast". Belle's love for books and knowledge could help Elsa understand more about her powers and the history of magic in the Frozen world.
Another plot could involve Anna getting a Miraculous. She could be a new hero in Paris, teaming up with Ladybug and Chat Noir. They might face a villain who has the ability to create eternal winter like in Frozen, and Anna's experience with dealing with Elsa's winter could come in handy. Also, the cultural differences between Arendelle and Paris could add some interesting twists to the story.
Well, perhaps in this fanfiction, the Organization XIII has a plan to use Elsa's powers for their own evil purposes. They manipulate events in Arendelle to make Elsa feel isolated and out of control. Sora, Donald, and Goofy show up and befriend Elsa, showing her that she can trust others. They then have to battle the Organization XIII members while also dealing with the internal struggles of Arendelle's characters, like Anna's determination to save her sister no matter what.
One idea could be that Elsa's ice powers attract the attention of the vampires in Diabolik Lovers. Maybe they see her as a unique being and try to either control her powers or use her for their own dark purposes. For example, one of the vampire brothers might think that her ice could be used to create a new kind of weapon or to enhance their lair's defenses.
Well, a really cool plot could be that the Frozen world is actually a long - lost planet in the Star Wars galaxy. The characters from Frozen get involved in the Galactic Civil War. Maybe Kristoff, who is used to cold environments, helps the Rebel Alliance on an ice - covered planet. Elsa could also use her ice powers to create defenses against the Empire's attacks, like building ice walls around Rebel bases. And Anna could befriend some of the alien species in the Star Wars universe and convince them to join the fight against the Empire.
A possible plot is a crossover with the story of Beauty and the Beast. The characters from Wonderland might enter the Beast's castle. Alice could befriend the Beast and try to break the curse using her Wonderland knowledge. Or, a crossover with the Snow Queen's realm. The cold and ice of that world would contrast with the colorful and chaotic Wonderland. The characters would have to find a way to survive in the frozen land while also dealing with the Snow Queen's power. Also, a crossover with the Little Mermaid's underwater world. The Wonderland characters would have to figure out how to breathe underwater and interact with mermaids and sea creatures.