Is the story set in late nineteenth century and has Watson and Holmes dealing with natural events and people? Then it is probably a straight pastiche. Are there elves or robots or aliens? Then it falls under everything else :D
To be fair, the line is a fairly unclear one. The Adventure of the Creeping Man from the original cannon kind of crosses the line. The Basil Rathbone movies I’ve watched seemed to be set later but are otherwise not fantastical. On the other hand, the Downey Junior ones are in the right time but have some fantastic elements. But too fantastic?
Okay. True confession time. I prefer the more fantastic stories. One of my first real introductions to Sherlock Holmes, before I had more more than a couple of the original stories, was Sherlock Holmes Through a Time and Space, edited by Isaac Asimov.
Beyond the fact that I happen to love fantasy and science fiction, I think more fantastic pastiches are a little more forgiving. If Sherlock Holmes is a robot or a alien teddy bear or investigating a were-automobile (yes, all examples from the collection :D), you’re not expecting the stories to read like they were written by Arthur Conan Doyle.
That said, I have come to really enjoy well written legitimate pastiches. I partially blame August Derleth and his Solar Pons. Openly written out of a love for Sherlock Holmes, they aren’t perfect but they are a lot of fun and made me appreciate Derleth a lot more than I had before.
One book I stumbled upon and rather enjoyed that I have to comment on is Conned Again, Watson. It’s actually a series of math lessons but written as Sherlock Holmes stories. I found it entertaining and educational. Not brilliant and not for a serious student of math but it stuck with me!
Sherlock Holmes is at home everywhere.