One interesting plot could be that a dark force in 'Once Upon a Time' decides to use the power of the vampires and werewolves in Twilight to take over both worlds. The characters from both universes would have to team up to stop it. The heroes like Emma from 'Once Upon a Time' and Edward from Twilight would lead the charge. They would have to learn about each other's powers and weaknesses quickly to be effective.
Well, the characters from 'Once Upon a Time' are often from fairy tales and have their own set of magic and personalities. The Twilight characters are in a more modern, vampire - werewolf - human love triangle world. If they were to interact, the vampires in Twilight might be intrigued by the dark magic in 'Once Upon a Time'. For example, Edward might be curious about Regina's use of magic and how it differs from his own vampiric abilities. And the werewolves in Twilight could potentially find common ground with some of the more animal - like characters in 'Once Upon a Time'.
A possible interesting plot could be that an ancient evil has awakened in the Once Upon a Time world. This evil is so powerful that it starts to corrupt the very fabric of the fairy - tale world. Hellboy is summoned through a magical portal because his unique nature, being part - demon, gives him some immunity to the evil's corruption. He teams up with Snow White and Prince Charming, who are initially skeptical of him but soon realize his value. Together, they embark on a journey to find the source of the evil and destroy it before it consumes both their worlds.
Well, it might be that the Avengers get transported to the Once Upon a Time universe. Thor could mistake the Evil Queen for Loki in a way, since she's so devious. And then you could have Black Widow using her spy skills to figure out the political intrigue in the fairy - tale kingdoms. The Hulk could be like a giant ogre in that world, and the heroes from Once Upon a Time would be both scared and intrigued by his power.
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.
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 idea could be exploring the backstories of the lost boys in more detail. Maybe one of them has a hidden connection to a character from the outside world. Another plot could involve a new villain coming to Neverland who has a different kind of magic that challenges Peter Pan. Also, a story about Wendy's granddaughter returning to Neverland to find a lost family heirloom could be interesting.
One possible plot could be that Mulan, instead of joining the army as a man, is visited by a magical fairy who gives her the ability to transform into a man at will. This way, she can still protect her family and her village without having to completely hide her identity. She then uses this new power to outwit the invaders and becomes a hero not just for her physical strength but also for her unique magical gift.
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.
In a Frozen - Once Upon a Time crossover, Elsa's cautious and self - isolating personality would be really interesting when combined with the world of Once Upon a Time. She might initially be wary of the new characters, but as she realizes they also face struggles, she could open up. Anna's eagerness to make friends and her loyalty would blend well with characters like Belle, who also values friendship. Kristoff would probably get along with the more down - to - earth characters. His no - nonsense attitude could be refreshing in the often - magical and complex world of Once Upon a Time.
I think Supergirl and Emma Swan would have an amazing dynamic. Emma is a strong - willed and independent character in Once Upon a Time. She has her own set of powers and abilities. When she meets Supergirl, they could bond over their shared experiences of being different and having to fight for what they believe in. They might also learn from each other's fighting styles and problem - solving techniques. Also, Supergirl and Hook could have an interesting interaction. Hook is a charming but also a bit of a rogue character. His swashbuckling ways would contrast with Supergirl's more straightforward heroism, and they could get into some fun adventures together.