Perhaps there could be a story where the doppelgängers concept from The Vampire Diaries is somehow related to the clones or alternate versions of characters in Smallville. It would be fascinating to see how the two different takes on 'doubles' interact. Maybe a doppelgänger from The Vampire Diaries has powers similar to those in Smallville due to some strange connection between the two universes, and they have to team up to solve a mystery that spans both their worlds.
One idea could be that a vampire from The Vampire Diaries somehow ends up in Smallville. Maybe they're drawn there by the strange meteor - related powers in Smallville. Another could be a character from Smallville, like Clark, having to deal with the vampire lore and politics of The Vampire Diaries' world. It could create really cool scenarios where Clark's powers are put to the test in a different way, like against a vampire's super speed or mind control abilities.
A crossover could involve a witch from The Vampire Diaries going to Smallville. In Smallville, there are all sorts of strange phenomena, and the witch could be either intrigued or try to use it to her advantage. For example, she might try to harness the power of the kryptonite. Meanwhile, the characters in Smallville would be baffled by her magic. This would lead to some interesting interactions as they try to figure out each other's abilities and motives.
Perhaps a spell gone wrong in the Buffy universe causes some of the main characters from there to swap bodies with characters from The Vampire Diaries. This could lead to humorous and tense situations as they try to figure out how to reverse the spell while still dealing with the threats in each other's worlds.
A cool concept would be to have Caroline from The Vampire Diaries go to New Orleans and get involved with the Mikaelson family. She could use her vampire strength and her knowledge from Mystic Falls to help them out in some way. Maybe she becomes friends with Hayley and they work together to keep the Mikaelson family safe from their enemies. Also, Damon could end up in New Orleans and cause some chaos among the Originals, as he is known for his unpredictable behavior.
One idea could be a story where the vampires from 'True Blood' and 'The Vampire Diaries' have to team up to fight a common enemy, like a powerful ancient vampire or a new type of supernatural threat. Another could be a love story between a character from 'True Blood' and one from 'The Vampire Diaries', exploring how their different vampire cultures affect their relationship.
Well, perhaps Hermione is sent to the 'Vampire Diaries' world to retrieve a powerful magical artifact that has somehow ended up there. Along the way, she meets Damon and Stefan. She forms an unlikely friendship with them as they help her search for the artifact. In return, she uses her magic to protect them from some of the more dangerous supernatural threats in their world.
One idea could be that some of the Glee characters visit Mystic Falls for a singing competition. While there, they get caught up in the vampire drama. Maybe a vampire takes a liking to Rachel's voice and wants to turn her into a vampire so she can sing forever. Another plot could involve the vampires using the Glee club as a cover to hunt during their events.
One idea could be that the Winchesters end up in Smallville while hunting a demon that has some connection to LuthorCorp's experiments. Maybe the demon is using the kryptonite there for its evil deeds. Another could be that Clark Kent discovers the existence of the supernatural world through Sam and Dean. He could initially be skeptical but then realize there are threats that even his powers might not be enough to handle.
One idea could be to cross it with another supernatural show like 'Teen Wolf'. The characters from both could team up to fight a common enemy, like an ancient and powerful vampire - werewolf hybrid. For example, Elena could use her charm and vampire skills, while Scott McCall could lead his pack in the fight.
One idea could be having Elena from 'The Vampire Diaries' meet Bella from 'Twilight'. Maybe they discover that they have more in common than they thought, like their love for vampires who are trying to resist their more primal instincts. They could team up to help their vampire boyfriends deal with some common threats in both their worlds.
One idea could be that the Hogwarts students are sent to Mystic Falls for a magical exchange program. Harry could meet Elena and they discover a new dark magic that is related to both the wizarding world and the vampires. Another could be that a vampire from Vampire Diaries somehow ends up at Hogwarts and has to navigate the magical rules and classes while hiding their true nature from most students. A third idea is that the Death Eaters team up with some of the bad vampires in Vampire Diaries to create chaos in both worlds.
Imagine if Elena Gilbert was somehow connected to an Infinity Stone. This would draw the attention of Thanos, but also the Avengers. The Vampire Diaries characters would have to protect Elena while also dealing with the fact that the Avengers are now in their small town. There could be a lot of mistrust at first, but as they learn more about each other's motives and capabilities, they form an alliance to keep the Infinity Stone out of Thanos' hands.