Doctor Who: 10 Great Ideas That Went Absolutely Nowhere
3. Captain Jack’s Missing Years
John Barrowman might be persona non grata following accusations of inappropriate on-set behaviour, but Captain Jack Harkness remains one of the most beloved creations from the NuWho era.
An immortal conman from many centuries in the future, a Jack appearance alongside the Ninth or Tenth Doctors was always a treat. He even resurfaced during the Thirteenth Doctor's tenure, busting her out of space prison.
As explored in the Torchwood series, Jack is a multi-faceted character who has done (and seen) a lot across his never-ending life. But he's actually missing two entire years of his memory, as explained in the episode The Doctor Dances.
The years were wiped from his mind by the Time Agency, Jack's former employers. Despite this posing some very intriguing questions about the future Face of Boe, the character has never seemed too interested in finding out more about them.
Jack doesn't even pursue these memories in the Big Finish audio series, which usually mops-up loose ends like some sort of plot janitor.
And with Barrowman likely out of the picture when it comes to Doctor Who, those two years might just remain a mystery for the rest of Jack's life - so basically, forever.