One idea could be that the characters from TVD end up in the PLL world due to a magical mishap. For example, Damon and Stefan could find themselves in Rosewood and get involved in the mystery of 'A'. They might use their vampire skills to protect the PLL girls from the unknown threats.
A cool concept would be a joint mission. The Supernatural side has to deal with a powerful, ancient evil that has some connection to the vampire origin in the TVD world. So, they team up. Dean might be initially hesitant about working with vampires like Stefan and Damon, but they soon realize they need each other's skills to defeat this new threat.
You can start by identifying the common elements between the two shows. For instance, both have a lot of mystery and secrets. Combine the 'A' mystery from PLL with the vampire - related secrets in TVD. Maybe 'A' is actually a vampire or is being controlled by a vampire. This would immediately create a high - stakes situation.
One idea could be that a vampire from TVD ends up in Riverdale. Maybe Damon Salvatore finds himself in the small town for some mysterious reason. He could get involved with the mystery-solving that the Riverdale gang is always up to. He might use his vampire charm and powers to help or sometimes complicate things. Another plot could be that a character from Riverdale discovers the existence of vampires and werewolves like in TVD. For example, Jughead could be intrigued by these supernatural beings and start writing a story about them, which then draws the attention of the TVD characters.
Perhaps a fanfiction where an ancient spell gone wrong results in multiple characters believing they are mates with one person. This would cause jealousy, confusion, and lots of power struggles as they all vie for the attention of this one individual. For example, Damon, Stefan, and Klaus all thinking they are mated to Elena in some strange new supernatural way.
You could do a crossover within different fable universes. For instance, take the moral - driven characters from Aesop's fables and put them in the world of Native American fables. The animals in Aesop's fables might learn new values from the Native American fable characters. Also, consider mixing fables from different cultures, like Asian fables and European fables, in a fanfic.
Maybe a crossover with a fantasy world where the Darkness entities interact with magical creatures. For example, they could form alliances or rivalries with elves or wizards, which would lead to a whole new set of adventures and conflicts.
A uzumaki crossover fanfic could involve crossing over with 'Bleach'. Picture an Uzumaki character in the Soul Society, their strong chakra being mistaken for reiatsu. They could bring new fighting styles and techniques to the Bleach universe.
One idea could be a crossover with 'Naruto'. Gon could meet Naruto and they could have a friendly competition in terms of strength and speed. Another idea is a crossover with 'One Piece', where the Hunters from 'HxH' join the Straw Hat Pirates on an adventure. Maybe they could be searching for a rare treasure that has both the allure of the Grand Line and the mystery of the 'HxH' world.
One idea could be a Dalek crossover with the Doctor Who companions. Imagine the Daleks facing off against a group of companions who have to use their unique skills and knowledge to outwit the Daleks. Another could be a crossover with a science - fiction adventure like Star Trek. The Daleks' aggressive nature would contrast greatly with the more diplomatic Starfleet.
One idea could be a crossover with 'Friends'. Jerry could visit Central Perk and have a stand - up comedy session there. His unique sense of humor might either amuse or confuse the 'Friends' gang.