Doctor Who has given fans tons of complex and compelling characters, many of whom have featured in a major narrative arc alongside the Doctor. Several are around for one adventure before they move on, although some have returned as ongoing companions or perhaps even antagonists.
With so many great characters to revisit down the line, the show won't be able to highlight all of the fan favorites. Yet, some of these individuals are so entertaining that they could have their own spin-off series, which may continue their storylines in unexpected ways.
Jenny
Jenny was the daughter of the Doctor, introduced during the Rusell T. Davies era of Doctor Who and portrayed by Georgia Tennant. The character seemingly died but would be revived thanks to her regenerative abilities and take off into the stars.
It's unclear whether Jenny is a true Time Lord or perhaps a hybrid of both human and Doctor DNA. Regardless, there's something very intriguing about an adventurer similar to the Doctor beginning their journey and finding their way in the galaxy for the first time — without a companion to guide them. Fans would certainly celebrate the return of the Doctor's daughter.
Parallel Earth Doctor
David Tennant's Doctor once used his severed hand in order to force a regeneration back into his body at the time. This consequently created an alternate version of the Doctor who had the physical elements of a human, down to one heart, but also the mind of a Time Lord.
The character went to the parallel Earth to live with Rose. It's the perfect way to bring back Tennant to the franchise, and a spin-off based on the character would surely help to recreate the heyday of Who with the additional twist of a Doctor forged by fire and war.
Martha Jones
Portrayed by Freema Agyeman, Martha Jones continues to be one of the best companions the Doctor has ever had. With advanced medical skills thanks to her previous work in a hospital, she has gone through a lot since joining team T.A.R.D.I.S. — even surviving a near-apocalypse by herself for years — and has helped to save humanity.
Martha went out into the world by herself leaving the Doctor behind and even marrying Mickey. She is now a veteran warrior and could certainly be featured in her own spin-off, which would see the character share both her empathetic abilities alongside her penchant for dispensing with aliens.
The Master
There have been so many fantastic stories featuring the Master in Doctor Who and yet fans are always clamoring for more from the Doctor's arch-nemesis. As a Time Lord, there have been countless variations of the character, each as devilish as the last. '
A spin-off show that works almost like an anthology, highlighting famous versions of the Master traveling in their own T.A.R.D.I.S. could be the anti-hero narrative fans are waiting for. It would be Doctor Who without any rules, with the dastardly plans of the Gallifreyan unleashed upon unsuspecting victims. It could also mix things up and include some kind of redemptive arc for the time traveler.
Rory Williams
Portrayed by Arthur Darvill, Rory Williams served as a companion alongside Amy Pond and Matt Smith's iteration of the Doctor. The character was hugely underrated until he was able to prove his heroism by becoming the man who waited; the Lone Centurion who sadly remained in place for centuries to be reunited with his family.
A lot could have happened during this period of time and only some of Rory's adventures were documented. A Doctor Who spin-off involving the travels and conflicts of the Lone Centurion that built up his legend would be a really fun premise to explore further.
Fugitive Doctor
Originally introduced as Ruth, the Fugitive Doctor is an unknown incarnation of the Time Lord, portrayed by Jo Martin, who demonstrated that the Doctor has been through further regenerations unbeknownst to both the character and the audience.
Partnered with a companion of her own, Lee Clayton, this Fugitive Doctor could have gone on plenty of missions that have now been wiped from the records. A spin-off miniseries could perhaps highlight some of those stories so that fans can get a better idea of the previous traveler behind the controls of the T.A.R.D.I.S.
Ashildr
Ashildr was an ongoing character, portrayed by Maisie Williams, who was an eternal being, witnessing human history over thousands of years. The character is more of an anti-hero but finished her journey traveling with Clara Oswald into the unknown.
It's unclear what happened to the duo after this, but a spin-off exploring either Ashildr's time on Earth or her travels after gaining her own T.A.R.D.I.S. would bring a compelling character and actor back to the franchise while opening up new storytelling opportunities.
Dream Lord
The Dream Lord, portrayed by Toby Jones, was supposedly a part of the Doctor's mind, tormenting him after a hallucinogen created a reality that Amy, Rory, and the Time Lord were trapped within. Yet, the Dream Lord seemed to have a bit more to him, as if he was an arch-nemesis from the past.
Perhaps the Dream Lord is indeed another Gallifreyan, and the Doctor lied about his origins. Played by the talented Toby Jones, a miniseries reintroducing the Dream Lord as another Time Lord like the Master could open up the lore of Who further while giving audiences a great antagonist.
Bill Potts
Portrayed by Pearl Mackie, Bill Potts helped Peter Capaldi's Doctor after the departure of Clara. The character had one of the saddest companion exits in Doctor Who, though, after only briefly appearing in the series, thanks to her conversion to a Cyberman. Her spirit would be lifted from the metal shell and she would go on new galactic adventures with her partner, a sentient oil alien from space.
Fans absolutely want to see what happens to Bill Potts next. What kind of trouble does she get into now she is visiting the rest of the universe and how does her new relationship with Heather influence her, now that she has been turned into an oil-like alien herself? These are big questions, but this huge change in the status quo is something a spin-off would be perfect for.
The Paternoster Gang
The Paternoster Gang is comprised of the Silurian Madame Vastra, her partner, the human Jenny, and the Sontaran Strax. Although they have vastly different points of view, these three detectives have solved plenty of mysteries without the Doctor.
There have been comic books and Big Finish Radio productions based on these fan favorites, but an ongoing spin-off series feels like the natural next step. Their combination of unique skills and diverse experiences makes them perfect alien explorers to take throughout space and time, as Sherlock-esque heroes.