Before you get too ahead of yourself: Matt Smith appearing on BBC’s Sherlock is nowhere near close to happening. In fact, Steven Moffat, showrunner for Sherlock and Doctor Who, may even be toying with the fans. (First rule of BBC: never trust Moffat.)
That being said, Moffat did fuel the crossover fire between the two successful series in a recent interview. (If you have any doubt about how much fans want a crossover between Doctor Who and Sherlock, try Googling “Wholock.”)
"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,” Moffat said of fans’ desire to see Smith go head to head with Sherlock star Benedict Cumberbatch.
However, Moffat wouldn’t necessarily ban Smith from auditioning or being cast in a non-time-lord role on Sherlock since he’s passed the Doctor torch to Peter Capaldi (cue the tears of mourning for Eleven).
"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,” Moffat said. “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."
While it’s not confirmed that Smith will ever appear on Sherlock, it’s also not confirmed that he won’t. Let the Wholock speculations continue with renewed fervor!