In keeping with past titles, they're likely plays on Arthur Conan Doyle stories, even if they don't always directly follow the plot. One of the original tales is called The Six Napoleons, which finds criminals destroying a bust of the man to find a hidden pearl. It And The Six Thatchers is already part of the extended canon of the show, appearing as a blog entry from Watson dated December 19.
Them there's The Lying Detective, which could have links to The Dying Detective, where Holmes pretends to be dying from poisoning in order to track down Culverton Smith. Could that be a clue to the villain that Toby Jones will be playing? We'll have to wait to find out the final title, but then waiting is all part and parcel of being a Sherlock watcher.
The new series will be on screens next year.