There's no shortage of demand for a Sherlock-Doctor Who crossover. There's an impeccable YouTube mashup dedicated it, GIFs on Tumblr fictionalizing it, and the Doctor himself even played Sherlock Holmes for a minisode. And now, there's a possibility that all this fangirling might turn into reality: Steven Moffat, who helms both Sherlock and Doctor Who, said "Nothing is impossible," when asked about the potential crossover by Zap2it.
It's not something we're particularly striving for. There's enough inevitable crossover talk between those two, it might be disconcerting for either of them to appear in the other's show. If there was a perfect part for him, there's no rule against it. We wouldn't want it to be stunt casting, because stunt casting doesn't work, so it would have to be just because he was overwhelmingly right for it. I mean, I love working with Matt. He's a phenomenal actor. I'd jump at the chance to work with him again. I don't know how likely that is to be on the set of 'Sherlock.'
So, you're saying there's a chance? After all, Matt Smith's tenure on Doctor Who has ended, and he's got nothing to do now except play Patrick Bateman in the American Psycho musical. And the crossover could occur at any point during the Doctor's timeline, so it's not like Smith's appearance wouldn't make sense now that he's off the show.
It's obvious that Moffat doesn't want to do anything too fanservice-y, but there's a lot of ways a crossover episode could be elegantly done. Not to mention that the ratings for a Wholock episode would probably break cable television.
But it's not a reality yet, so until then we'll just have to be satisfied with imagining it. And fortunately for us lazy people, the talented artists of Tumblr have already done it for us with some great fan art. Here are some of the best ones:
Sherlock and Watson as Bill and Ted on The Doctor's, er, "Telephone Booth"
The Bill and Ted's Excellent Adventure parody you never knew you wanted. It's the turducken of Sherlock-Doctor Who crossovers: a crossover within a crossover.
Doctor Who Takes Sherlock To Space, and He Is Not Impressed
Such a Cumberbetch.
Sherlock Tags a Dalek
Sherlock is even bored by the Doctor's sworn enemy.
Sherlock Argues with a Dalek
There's a surprising amount of Sherlock-Dalek interaction in this crossover universe, although the Dalek's side of the conversation is most likely just "EXTERMINATE!"
Sherlock and the Doctor Switch Companions
An interesting concept for a crossover episode. Do you see the potential now, Moffat? A Wholock episode doesn't just have to be all stunt casting and fan-service.