A really cool fanfiction idea could be that Naruto and Elsa are both summoned by a powerful sorcerer in a different dimension. They have to work together to find a way back home. Along the way, they discover that their powers are connected in some way, like Naruto's chakra can enhance Elsa's ice powers and Elsa's ice can be used to create new jutsu for Naruto.
Another concept could be that a powerful ancient evil is affecting both the Percy Jackson universe and Arendelle. Elsa and Percy are the only ones who can sense it at first. They must team up, traveling between their two worlds, to find the source of the evil. Along the way, they learn about each other's powers and cultures, and their combined powers are the key to defeating the evil.
A possible plot is that Elsa's kingdom is threatened by a force that is related to the same kind of threats the Avengers face. The Avengers hear about her and her powers and travel to her world to offer their help. Along the way, they learn about her backstory, and she learns about their heroics. Together, they combine their powers to save her kingdom and in the process, Elsa decides to join the Avengers on some of their adventures in their own universe.
A plot idea might be that there is a strange blizzard that is not caused by Elsa. Kristoff, suspecting foul play, comes to Elsa for help. Together, they discover a dark sorcerer is behind it. As they battle the sorcerer, Kristoff helps Elsa see that she doesn't have to be afraid of her powers and can use them more freely. This could lead to a deeper friendship or more between them.
Well, perhaps there could be a fanfiction where Hiccup and Elsa are both summoned by a powerful wizard to help with a prophecy. They don't trust each other at first, but as they face the challenges set by the wizard, which include tasks that require both fire (from Hiccup's dragon) and ice (Elsa's power), they start to respect and care for each other.
Another interesting plot could be that the reader is a time - traveler from the future. They come to Arendelle during Elsa's reign and know about all the events that are going to happen. The reader tries to help Elsa avoid some of the hardships she will face in the future, but in doing so, they accidentally change the course of history. This creates a complex situation where Elsa and the reader have to work together to set things right while also dealing with their growing feelings for each other.
One idea could be that Dark Elsa accidentally freezes Anna's heart again, but this time it's a different kind of ice that can only be thawed with a magical item hidden in a far - off land. So, Anna has to embark on a dangerous journey while Dark Elsa struggles with her guilt.
One idea could be that Elsa discovers she has the power to create a magical baby snow creature, and Jack Frost comes to visit and is fascinated by it. They then start to explore the possibilities of Elsa's powers together in relation to this new creation.
One idea could be that Elsa discovers she has vampire powers after being bitten in a hidden part of the frozen forest. Another could be a love story where a vampire falls for Elsa and tries to convince her to embrace the dark side while she struggles with her own ice powers and the implications of becoming a vampire. Or perhaps Elsa is turned into a vampire and has to learn to control her new thirst while still ruling Arendelle.
A third idea is a time - travel based fanfiction. Elsa somehow gets sent back in time to the era of Harry Potter's parents. She uses her powers to save them from Voldemort, which changes the entire future of the wizarding world. Harry then meets Elsa in his own time, and they have to figure out how her actions in the past have altered the present and what they can do to keep the peace in this new, changed world.
One idea could be that a powerful dark sorcerer kidnaps Elsa because he wants to use her ice powers for his own evil deeds. He locks her in a magically sealed dungeon deep in an ancient castle. Another plot could involve a group of thieves who mistake Elsa for a valuable princess they can ransom. They take her to a hidden lair in the mountains. A third option is that a jealous rival from another kingdom, who envies Elsa's powers and her popularity, arranges the kidnapping to discredit her.