There have been speculations that "Sherlock" and "Doctor Who" would have a cross-over since creator Steven Moffat manages both shows. There are also whispers of the possibility of a cross-over since The Doctor (Peter Capaldi) can travel through time and space and it would be easy for him to go to Sherlock's place. 

Fans go as far as musing on possible scenarios as to how the characters would meet. According to Unreality TV, Sherlock could be solving a case like a charity fundraising spoof such as Children In Need.  The Doctor would be a pertinent character who holds the clues or messages about the case. 

At first, Moffat was against the idea, saying that although he likes a cross-over, he thinks that it is impossible to produce. 

"It can't happen. There are certain rules I cannot break," he announced in Radio Times. 

However, he flipped his statement at an event held by the Royal Television Society earlier this week. He said that he is in favor of doing a cross-over between the two shows and added that it is his "Sherlock" co-creator Mark Gatiss and the rest of cast and crew who are against the idea. 

He explained that Sherlock Holmes already existed in "Doctor Who" series and that The Doctor has disguised like him in the past. But, despite his agreement, he still considers the idea an impossible thing. 

Meanwhile, Gatiss revealed that the Christmas special slated for December 2015 has started its pre-production and announced that it will be a "very special special." Previous reports claimed that the storyline will be based on "The Adventure of the Blue Carbuncle." The filming of the special episode is reported to start in January. 

"Sherlock" season 4 is returning to BBC in early 2016 with no official date yet. The fourth season is set to have three episodes.