For a PLL and Vampire Diaries crossover fanfiction, you could start with a spell gone wrong in Rosewood that transports some of the PLL characters to the world of The Vampire Diaries. They might be mistaken for new threats or potential allies. The liars would have to adapt to the new rules of a world filled with vampires, werewolves, and witches. Hanna could befriend Caroline for her fashion sense and they could end up sharing secrets about their respective worlds. Spencer would be intrigued by the history and lore in Mystic Falls and start researching how to get back home while also getting involved in the local mysteries.
One idea could be that the Pretty Little Liars characters find themselves in Mystic Falls. Maybe A is somehow related to the supernatural threats in The Vampire Diaries. The liars could team up with the Salvatore brothers to figure out the mystery behind A while also dealing with vampires and witches.
To write a PLL - Gossip Girl crossover fanfiction, think about the different settings. Rosewood is a small town compared to New York. You could have the Gossip Girl characters visit Rosewood for a change of scenery. Chuck could try to take over some local business in Rosewood and the PLL characters would have to stop him. Meanwhile, the Gossip Girl drama would spill over into Rosewood as the PLL girls get caught up in the love triangles and power struggles of the Upper East Side. The key is to blend the mystery and drama elements from both shows.
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.
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.
Well, a great plot could be that Caleb finds a strange encrypted file on his computer that seems to lead to a hidden treasure in Rosewood. As he tries to decipher it, he has to team up with some unexpected characters from the show. Maybe he also has a vision or a strange dream that leads him to believe there's something supernatural going on in the town related to the file. Also, he could get into a situation where he has to choose between his loyalty to his friends and his own personal gain from the treasure.
One interesting plot idea could be a battle between the hunters from Supernatural and the vampires from Vampire Diaries. Maybe the Winchesters end up in Mystic Falls and think all the vampires there need to be hunted. Another idea is a character swap. For example, Castiel could end up in the Vampire Diaries world and interact with the characters there in a very different way compared to his usual interactions in Supernatural. A third idea could be a joint mission to fight a greater evil that threatens both universes, like an ancient demon that wants to use the power of the vampires in Vampire Diaries and the magic from Supernatural for its own dark purposes.
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.
Klaus Mikaelson is definitely one of the most popular. His complex and often villainous nature in The Originals makes him a fascinating character to bring into the world of The Vampire Diaries. He can stir up all kinds of trouble and create new dynamics among the characters in Mystic Falls.
A popular one is 'A New Alliance'. In this fanfiction, the plot brings together the different supernatural elements from 'The Vampire Diaries' and 'The Originals' in a unique way. The author does a great job of maintaining the personalities of the original characters while also creating new and exciting storylines. It's a must - read for fans of the two shows.