Firstly, and possibly most importantly, the entire canon of Sherlock Holmes by Sir Arthur Conan Doyle, is now in the public domain. That means that you can write any Holmes story you want. (Since Winnie-the-Pooh and Cthulhu are also now public domain characters, I have seen some really weird mashups.) There have been anthology collections of Holmes stories published in the last few years.
Write Well. Publish Better.