Aside from the main series, Doctor Who has a wide variety of spin-off media, from comic books, to the Big Finish audio stories, to series such as Torchwood, The Sarah Jane Adventures and the short-lived Class. However, there is one more spin-off series that actually existed within the run of Doctor Who itself, one that is worthy enough for its own show.
Deep within the streets of 1800s London resides three extraordinary beings dubbed The Paternoster Gang. They are Madame Vastra, a Silurian; Jenny Flint, a human and Strax, a discharged commander of the Sontaran Empire. They were saved by The Doctor during varying times in their lives and were given a chance to live in the 1800s to solve crimes and hunt monsters.
They make their first appearance in the Series 6 episode "A Good Man Goes To War," wherein The Doctor takes up every favor he has to rescue Amy Pond and her baby. After the success of the adventure, most audiences assumed they would never see the Paternoster Gang again, but during the 2012 Christmas special, "The Snowmen," the gang makes their first official appearance as a team, showcasing their talent, friendship and unbelievable entertainment value.
Their cover story is that Madame Vastra is the madame of the 13 Paternoster Row manner with Jenny as her maid and Strax as her butler. In reality, all three coordinate to take down any evil beings that cause harm to their city while occasionally assisting The Doctor if and when he finds himself in their time. After the events of "A Good Man Goes To War," the gang continues to show up for a majority of the episodes within the second half of Series 7, becoming fan favorites.
The gang has the makings of the perfect spin-off, as a found family that solves crimes, with a sci-fi twist to Sherlock Holmes' style. The series even jokes that Sherlock's creator, Arthur Conan Doyle, based his characters on Vastra and Jenny. While little is known about their adventures outside of The Doctor's life, it's revealed Vastra's responsible for stopping London's most infamous killer, Jack the Ripper.
With all the pieces of the puzzle in place, it is odd that the showrunner at the time, Steven Moffat did not go through with creating the spin-off, as fans would have relished in a full run of the gang. In an interview with Vortex, Moffat stated he heavily considered producing a Paternoster series, but he was afraid of diluting the Doctor Who brand by having too much Who on television.
The Paternoster Gang makes their final appearance in Series 8's premier episode, "Deep Breath," where they help a newly regenerated Doctor find his footing. However, in 2019, the Paternoster Gang returned for a Big Finish four part anthology audio series titled The Paternoster Gang: Heritage.