Exec. Producer and Head Writer Steven Moffat recently revealed to SFX Magazine that he did indeed consider the idea of a spin-off series starring Madam Vastra and Jenny, the wildly popular guest characters from the Doctor Who mid-season finale "A Good Man Goes to War". He even pitched the idea to BBC Controller of Drama Commissioning Ben Stephenson, who said "Yes!"
Unfortunately, since Moffat is a very busy man, running both Doctor Who and Sherlock, we probably won't be seeing an actual Vastra/Jenny TV series. But the Moff is pleased with reaction fans had to the characters, so he says he'd certainly like to revisit them in the future. "I love that Victorian setting, and having kicked it out of Sherlock Holmes I can always kick it in somewhere else!"
