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.
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.
One common theme is the comparison between the two Sherlocks. Writers often explore how the Benedict Cumberbatch - like Sherlock from the BBC show and the Jonny Lee Miller - based Sherlock in Elementary differ in their deductive methods and personalities. Another theme is the interaction between the side characters. For instance, how Watson from each show would get along or clash.
The idea of a bigger, overarching mystery that requires both the skills of Sherlock and the resources of NCIS is also a common theme. It might be a case that has roots in both the UK and the US, or something that involves a very elusive criminal that only their combined efforts can catch. This theme allows for a lot of exciting plot development and showcases the strengths of both universes.
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.
Well, Jack Harkness from Torchwood would probably be quite intrigued by Sherlock's deductive abilities. He might initially test Sherlock, throwing out strange cases to see how he solves them. Sherlock, on the other hand, would likely be a bit put off by Jack's flamboyant nature at first but would soon realize his value in dealing with the more otherworldly aspects of cases.