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.
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.
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.
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.
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.
Well, one interesting plot point is the clash of the different vampire abilities. In Twilight, vampires have certain powers like super speed and strength, and in TVD, vampires have different powers like compulsion. When combined in fanfiction, seeing how these powers interact or conflict can be really exciting. For example, if a Twilight vampire tries to use their speed against a TVD vampire who can compel them, it creates a whole new dynamic.
One idea could be a crossover with Supergirl. They could team up to fight a common enemy like Lex Luthor who has some new, devious plan that threatens both Gotham and Metropolis. Another idea is a crossover with the Arrow. The two vigilantes could exchange fighting techniques and learn from each other's experiences in protecting their respective cities.
One idea could be a crossover with a popular fantasy series like 'The Lord of the Rings'. You could have planeswalkers from MTG enter Middle - earth. They might interact with the hobbits, elves, and dwarves. Another idea is a crossover with a superhero universe. Picture a planeswalker with superpowers like Superman or Spider - Man. They could team up with other heroes to fight a common enemy. Maybe a powerful Eldrazi that has invaded their world.
One idea could be a Spiderman and Iron Man crossover. Spiderman could team up with Iron Man to fight a new, powerful villain. Maybe they have to combine their tech and skills, like Spiderman using his web - slinging abilities along with Iron Man's high - tech gadgets to take on an enemy who can manipulate electricity.
A cool 'injustice crossover fanfic' could involve 'Injustice' and the 'Watchmen' universe. The gritty and morally complex world of 'Injustice' would contrast well with the unique and dark 'Watchmen' universe. Rorschach could find himself in the 'Injustice' universe and his uncompromising view of justice might clash with some of the characters there, while Dr. Manhattan's god - like powers could be a major factor in the story, either as a threat or an unexpected ally.
A great concept for a pokemon crossover fanfic is to combine it with the world of Studio Ghibli. Imagine Pokémon in the beautiful and magical landscapes of Spirited Away or My Neighbor Totoro. The Pokémon could interact with the unique characters in those stories. Also, a crossover with the Star Wars universe would be cool. Pokémon could be on different planets, and some might even have force - like abilities.