Steven Moffat is the big chief currently in charge of both Doctor Who and Sherlock. During an interview regarding Season 4 of Sherlock, EW asked him; "Last I checked, you were swayed that a Sherlock and Doctor Who crossover is not a good idea and won't happen. Any movement on that?"
Joey Paur, writing for GeekTyrant:
I can't help but feel that Joey is missing the point on this one. The cast and crew of Sherlock said it best when they pointed out that "it will never be as good as [the audience] think it's going to be". I admit that seeing Sherlock and The Doctor together would be an intriguing clash of minds, but ultimately I see two major problems with it;
1) It will never live up to expectations.
2) Having Sherlock exist in the Doctor's universe, with all the wonderful aliens and monsters that exist throughout (not to mention all the Christmas invasions) would completely undermine the universe in which Sherlock exists.
As much as I love both properties, I just can't agree with GeekTyrant on this one. For now, the amazing fan-made 'Wholock' short is good enough for me (and be sure to check out the VFX breakdown to truly appreciate how good the short is).