1. By reading the original stories by Arthur Conan Doyle you get to explore the mind of Sherlock Holmes in a way that the TV series and movies aren’t able to deliver. You get to understand the way Doyle uses language.
2. You could move Sherlock Holmes to another big city, you could make John and Sherlock have a dog. But because the people that will read your story will most of the time be Sherlock Holmes fans, you shouldn’t change that much more. If you do, you might ruin what people love about the original stories. And most important, truly think about it before you kill a major character, there’s a big chance that you’ll ruin the story by killing Mrs. Hudson for example
3. You need to do some research to find out if your ideas for a Sherlock Holmes story are original. You might find the idea of writing a story about Sherlock Holmes versus Hannibal Lecter, a character in a series of suspense novels by Thomas Harris, very good. However, a simple search on the internet will tell you that this idea has been worked out more than a thousand times.
4. Besides Arthur Conan Doyle there are many people who have written their own Sherlock Holmes stories, by reading those stories you will get in insight into how others have dealt with the characterisation, dialogue and plotting.
5. No over-the-top action
6. Sherlock Holmes should not have a woman he loves. Arthur Conan Doyle described Sherlock Holmes as “inhuman as a Babbage’s calculating machine” he felt his famous detective was not at all interested in romance. It does look like Sherlock has a love affair in the BBC series Sherlock, with a woman called Irene Adler. Then again it would be wrong to make Sherlock have another love affair because of the famous sentence “To Sherlock Holmes she is always the woman” and John Watson said: “All emotions and that one (love) particularly, were abhorrent to his (Sherlock Holmes) cold, precise but admirably balanced mind.”
7. The characters in Arthur Conan Doyle’s Sherlock Holmes stories are fictional. Quite some stories have been written in which Sherlock meets famous people, Nicholas Meyer made a novel in 1974 in which he puts Sigmund Freud and Holmes together called The Seven Percent Solution. Peter Macek wrote Sherlock Holmes and Hitler’s Messenger of Death, a book about Sherlock Holmes solving a case in which Hitler plays a big role. But as novelist writer Anthony Horowitz states: “The power of the books is that they largely create a world of their own with very little reference to contemporary affairs.” Arthur Conan Doyle uses very notable characters as The King of Scandinavia and Wilhelm Gottsreich Sigismond von Ormstein, but even these are fictional. Hence/So, no appearances of famous people in Sherlock Holmes stories you.