One idea could be that Harry and his friends get transported to a frozen wasteland during a spell gone wrong. They have to use their magic to survive the extreme cold and find a way back. Maybe they encounter magical creatures native to the frozen area that are both dangerous and helpful.
Another plot could involve a prophecy that ties the fate of the wizarding world to the frozen wasteland. A group of Hogwarts students, including Harry, are sent on a quest to the wasteland to uncover the meaning of the prophecy. Along the way, they meet a tribe of ice - dwelling wizards who have their own set of rules and magic. The students must learn from these wizards and work together to fulfill the prophecy and save both the frozen land and the wizarding world at large.
Well, a cool plot might be that Elsa's ice powers are actually a form of ancient magic that was lost in the wizarding world long ago. Harry and his friends figure this out and bring Elsa to Hogwarts to study her powers. Meanwhile, Anna follows and gets into all sorts of adventures at Hogwarts, like trying to fit in with the students from different houses. Also, the dementors could be attracted to Elsa's coldness, but instead of sucking out her soul, they end up getting frozen by her powers.
Some interesting Frozen 2 fanfiction plot ideas could revolve around the idea of a magical item. For example, there could be a magical crystal hidden deep in the forest that has the power to either save or destroy both the forest and Arendelle. Elsa and her friends embark on a quest to find this crystal before it falls into the wrong hands. Another idea is that a dark force from outside the known world tries to invade the forest and Arendelle, and the characters have to unite not only the people of Arendelle and the forest but also other magical beings to fight against it. Also, a plot could be centered around a prophecy that was never fully revealed in the movie, and Elsa and Anna must figure out what it means for their future.
In a frozen wasteland, the Hogwarts students would first need to find a way to protect themselves from the cold. Harry might remember some spells he learned in Defense Against the Dark Arts that could be modified for protection against the elements. Hermione would dig into her books - if she managed to bring any - or use her photographic memory to recall useful information. Ron would be the one to keep an eye out for any immediate threats. As they adapt, they might start to discover new magical properties in the ice and snow. For example, they could find that certain crystals in the ice can be used to enhance their spells, or that the snow can be magically manipulated in ways they hadn't thought of before. This would change their understanding of magic and how they interact with their environment.
A possible plot is that a dark wizard from the Harry Potter world tries to take over Arendelle. Elsa and Anna have to team up with Harry and his friends to stop this threat. The Frozen characters would be introduced to the concept of the Deathly Hallows and Horcruxes. Elsa's ice powers could be a key factor in fighting the dark magic. Maybe her ice can counter some of the dark spells, and together they all discover new ways to combine their unique abilities to defeat the evil wizard.
Well, in some fanfiction, we might see Anna and Kristoff visiting Hogwarts. They could be amazed by the moving staircases and magical creatures. In reverse, Harry, Ron and Hermione could end up in Arendelle. Hermione might be intrigued by the way Elsa controls her ice powers and try to find a scientific or magical explanation for it. Also, the Dementors from Harry Potter could be compared to the dark forces in Frozen 2, like the mist that Elsa has to face.
One interesting crossover could be a scenario where Hermione uses her magic to help Elsa control her powers better. Maybe she creates a spell that can soothe Elsa when she's about to lose control.
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.
A great plot for a Potter Manor fanfiction could involve a mystery that surrounds the manor. For instance, strange noises at night lead Harry to uncover a long - forgotten chamber. It could also be about Harry restoring the manor to its former glory, perhaps with the help of Hermione's research on ancient magic used in the construction of such manors.
A traveler could be from a different magical realm entirely and come to the Harry Potter universe. They bring with them unique magical abilities that both fascinate and frighten the wizards in the Harry Potter world. This could lead to a series of adventures where they either share their knowledge or are hunted down by those who fear the new magic they possess.
One idea could be that Hecate, being the goddess of magic, takes Harry Potter under her wing secretly to teach him ancient and powerful magic that Dumbledore doesn't know. This could lead to Harry having new abilities to fight Voldemort.