One interesting plot could be Batman and Sherlock teaming up to solve a mystery that involves both Gotham and London. Maybe a criminal organization is operating in both cities and they have to combine their skills. Batman's combat and detective skills in Gotham's dark alleys and Sherlock's deductive reasoning in London's Victorian - like settings would make for an exciting story.
Batman is very much a lone wolf, while Sherlock often has Watson by his side. Their personalities would initially clash because Batman is not used to having someone so talkative and inquisitive around all the time. But as they start to solve the mystery together, they would find that their different approaches to problems are actually very complementary. Batman's more hands - on approach combined with Sherlock's intellectual approach would make them a formidable team.
The theme of justice is also very common. Both Sherlock Holmes and Batman are dedicated to bringing criminals to justice. In fanfiction, they often team up or compete in the pursuit of this goal. They might have different ideas about what justice means exactly, but they both strive to make their respective worlds a better place by stopping the bad guys.
First, know both universes well. If it's a Sherlock and Harry Potter crossover, understand the rules of magic in HP and Sherlock's deductive methods. Then, create a unique plot. Maybe Sherlock is called to Hogwarts to solve a mystery that baffles the wizards. Also, keep the characters in - character. Sherlock should still be his brilliant, aloof self. And don't forget about good grammar and spelling.
Maybe there's a mystery that spans both London and Gotham. A villain is using a new kind of technology or drug that was first experimented with in London and is now being exploited in Gotham. Sherlock Holmes, having heard about Batman's reputation for dealing with such matters, travels to Gotham. They meet and initially clash due to their different methods. But as they start to share information, they realize that the villain is someone they both have a history with in some way. They then combine their efforts, Holmes with his mind and Batman with his gadgets and physical prowess, to stop the villain.
One popular idea is a Sherlock crossover with the Harry Potter universe. Imagine Sherlock using his deductive skills in Hogwarts to solve magical mysteries. Another is a crossover with the Marvel universe, where Sherlock might team up with Iron Man to outwit villains using their combined intelligence. A third could be a crossover with the Doctor Who series, with Sherlock and the Doctor traveling through time and space while solving complex cases.
Yes, there are many. Some might explore how Sherlock's deductive skills would handle the supernatural elements in the Supernatural universe. For example, he could try to find a logical explanation for the existence of demons or ghosts, which would create an interesting contrast with the Winchester brothers' more traditional hunting approach.
A common theme is the exploration of power. Sherlock has his mental power and deductive abilities, while the Supernatural characters have powers like magic and the ability to fight demons. In fanfiction, they might combine their powers to face a greater threat. For example, Sherlock's ability to analyze a situation could help the Winchesters in a battle against a powerful supernatural entity, and the Winchesters' knowledge of the supernatural could help Sherlock understand things he couldn't before.
One common theme is the clash of their personalities. Sherlock's cold logic and Hannibal's refined and sometimes sinister charm create a lot of tension. For example, in many fanfics, they initially distrust each other due to their very different approaches to life and crime - solving.
Mystery is also a big theme. Since Sherlock is known for solving mysteries, fanfiction often has him using his skills to solve some big mystery in Middle - earth, like a disappearance or a strange occurrence that's affecting the Hobbits or the whole of Middle - earth.