9. What Happened To Susan?
When the First Doctor left his granddaughter, Susan, in 22nd century London at the end of The Dalek Invasion of Earth, he promised that “one day” he would come back. Decades later, we’re still waiting to see him make good on that promise.
That said, continuity-wise, it might be more trouble than it's worth to bring Susan back on TV. That’s because she has in fact been reunited with her grandfather (or grandmother) in other mediums.
On audio, Susan has enjoyed numerous adventures alongside the Eighth Doctor, and we even met her son (the Doctor’s great-grandson), Alex. Meanwhile, in prose, the 2020 short story Fellow Traveller saw Susan come face-to-face with the Thirteenth Doctor.
Most recently, Big Finish has covered the two extremes of Susan’s life, with 2021's After the Daleks detailing the immediate aftermath of the Daleks’ occupation, and 2020's Susan’s War exploring her involvement in the Time War.
Continuity issues aside, it would be brilliant see Carole Ann Ford reprise the role one more time - especially given that the other surviving member of the original TARDIS team, William Russell, recently returned to the show.
RTD's even been teasing Susan's return, so it's not out of the question...