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.
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.
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'.
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 possible plot could be that the vampires from Twilight somehow end up in the world of Glee. Maybe they are drawn there because of a unique musical energy. The Cullens could enroll in McKinley High and join the Glee club. Edward, with his musical talent, could be a star singer, but his vampire nature would cause some interesting conflicts, like how to handle his super strength during dance numbers or when handling musical instruments without breaking them.
Well, perhaps there's a prophecy in the Percy Jackson world that involves the Twilight vampires. Percy and Annabeth travel to Forks to find out more. They get caught up in the love triangle between Bella, Edward, and Jacob. At the same time, a dark force from the underworld is starting to rise and it's affecting both the demigods and the vampires. So they have to put aside their differences and use their combined powers to stop it. This would involve a lot of character development as they learn from each other's strengths and weaknesses.
One possible plot could be that Wolverine somehow ends up in the Twilight universe. Maybe he's on a mission that goes wrong and he gets transported there. He then meets the vampires and werewolves in Twilight. The vampires might be intrigued by his healing factor and his adamantium claws. And the werewolves could see him as either a threat or an ally. He could get caught in the middle of the conflict between the Cullens and the Volturi, using his combat skills to tip the balance.
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 interesting plot point is the clash of different supernatural powers. For example, the vampires' strength and speed in Twilight against the various abilities of the hunters in Supernatural like exorcising demons. Another is the cultural shock between the two universes. The vampires in Twilight have their own society and rules which are very different from the world of Supernatural.
One plot idea could be that Bella Swan, from Twilight, gets lost in Gotham City. Batman discovers her and at first suspects she's part of some new vampire plot in his city. But as he gets to know her, he realizes she's just a lost girl with her own set of supernatural problems. Another idea is that the Cullens move to Gotham for some reason. Edward and Batman cross paths and Edward's mind - reading ability both intrigues and annoys Batman. They could end up working together to stop a new breed of super - vampires that are terrorizing the city.