Well, a great crossover idea in these fanfics is the concept of the characters' songs. For example, Elsa's 'Let It Go' and Belle's 'Beauty and the Beast' song. Imagine if the two stories collided and there was a moment where both characters sang their iconic songs in a combined musical number. It would be a very emotional and powerful moment. Also, the idea of the dwarfs from Snow White (since Frozen and Beauty and the Beast are both Disney) helping out in the Beast's castle could be really fun. They could add some comical relief to the more serious situations in the story.
In these fanfics, one really cool crossover is the use of the magic mirror from Beauty and the Beast in the Frozen world. It could be used to find out more about Elsa's powers or her past. On the other hand, the reindeer Sven from Frozen could visit the Beast's castle and have some sort of adventure there. The Beast might be initially suspicious of this strange animal but then grow to like him. And then there's the possibility of the fairies from Sleeping Beauty (again, in the Disney universe) getting involved in the story. They could help Belle and Elsa in their quests, whether it's breaking the Beast's curse or helping Elsa control her powers better.
In a fanfic crossover, we could see Tadashi from Big Hero 6 and Elsa having a conversation about their responsibilities. Tadashi was responsible for Hiro and his growth, and Elsa is responsible for Arendelle. They could share their experiences of feeling the weight of their duties and how they overcame their insecurities to fulfill them. This could be a really deep and emotional part of the crossover story.
One interesting crossover could be Rarity from My Little Pony in the role of Belle. Rarity loves beautiful things and is very refined, much like Belle. She could bring her sense of style to the Beast's castle and maybe even design new outfits for the enchanted objects. Another idea is that Twilight Sparkle could use her magic to try and break the curse in the same way Belle's love did. The ponies' world of magic and friendship could add a whole new dimension to the Beauty and the Beast story.
One interesting crossover could be the character designs. For example, taking the unique art style of Cuphead and applying it to the Undertale characters. Sans could have a Cuphead - like appearance with a big head and small body, while still maintaining his laid - back and mysterious personality. Another aspect could be the gameplay elements. Maybe in the fanfic, the battle system of Undertale is combined with the challenging boss fights from Cuphead. The soul - collecting mechanic of Undertale could be incorporated into a Cuphead - style level where the protagonist has to avoid attacks and collect souls at the same time.
A long - exposure fanfic might also explore the idea of Beauty bringing life back to the castle. With her presence, the long - forgotten gardens start to bloom again, and the dusty halls regain their former glory. This could be a metaphor for how she is bringing light and hope into the Beast's life, and how their relationship is slowly transforming both of them.
One interesting crossover could be the magic elements from GOT in the Naruto world. For example, the use of Valyrian steel could be equated to some rare ninja weapons. Maybe the White Walkers could be a new threat in the Naruto universe, and the ninjas would have to use their skills like jutsus to fight them.
One interesting crossover could be Naruto learning time - stop abilities from Star Platinum. It would completely change the dynamics of his battles in the Naruto universe. He could use it to outmaneuver his opponents, like Sasuke or Orochimaru.
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.
One interesting crossover could be Elsa using her ice powers in the Hunger Games arena. She could create ice barriers for protection or freeze her opponents' weapons. Another could be Anna's fearless nature helping her navigate the deadly Hunger Games, forming alliances with other tributes. Maybe characters from Frozen could interact with the Capitol, like Elsa impressing them with her powers during an interview.
In a Percy Jackson - Frozen fanfiction, perhaps Annabeth could meet Anna. Annabeth would be impressed by Anna's determination and loyalty. They could go on an adventure in the Arendelle kingdom, with Annabeth using her strategic mind to solve problems that the Frozen world is facing, like a strange curse that is making the ice in the kingdom melt uncontrollably.