Well, this is interesting.
Though I’d much rather see John Munch turn up in an episode of Doctor Who, I have to admit that the idea of the Doctor and Sherlock Holmes crossing paths isn’t a bad one. With the Canonical Holmes, there’s Patricia Bow’s “The Captive Sleuths,” Andy Lane’s All-Consuming Fire, and probably more fanfic that you can shake a stick at. And for Sherlock, there’s probably way more fanfic than anyone could read in a lifetime.
And, Matt Smith himself says, “I think Mark and Steven kind of hate the idea of Doctor Who and Sherlock ever meeting.” Doctor Who inhabits a world where Daleks invade Earth every June or November, and really bad things happen every Chrsitmas Day. Sherlock inhabits a world of ratiocinative powers. The two worlds don’t easily coexist.
Yet, if I were the one in charge of a Doctor Who/Sherlock crossover, I’d play up the vast differences, in an unexpected way.
I’d do two episodes, one Who, one Sherlock.
These separate episodes would overlap to some extent but never have the protagonists of each quite meet. The Doctor would be involved in an adventure in modern day London with a guest cast in his episode, Sherlock would be involved in a mystery with the same guest cast in his episode, and if you watched them together you’d get the entire story, but if you watched them separately they would work entirely as Sherlock or Doctor Who stories.
Maybe the closest the protagonists come to meeting is when the TARDIS departs — at the end of the Doctor Who episode, we see the TARDIS dematerialize and then Sherlock rounds a corner to see nothing, while the Sherlock episode gives us the same thing from Sherlock’s perspective (because maybe he’s tracking the Doctor’s mysterious footprint of something).
I could see that kind of story appealing to Moffat — it would be a puzzle, and Moffat loves his puzzles — though it wouldn’t satisfy the fans who want to see Smith and Benedict Cumberbatch chewing scenery together.
The puzzle thing doesn’t mean I expect it to happen, though.
And I still want Munch on Who, dammit!