Captain Jack Harkness
If we had to pick just one character to return and meet Jodie Whittaker’s Doctor, we would probably choose Captain Jack Harkness. The immortal Time Agent hasn’t been seen on TV since 2011 yet he’s still remembered fondly as one of the modern series’ best creations. Fans have repeatedly called for him to return opposite either Matt Smith or Peter Capaldi’s Doctors but, sadly, it wasn’t to be. That has to change with the Thirteenth Doctor, though.
It’s definitely not actor John Barrowman who’s keeping Jack from returning. He’s kept himself busy by appearing as Malcolm Merlyn in Arrow for the past few years but he’s always found time to lend his voice to numerous audio productions. He’s also been very public about his efforts to get his spinoff show Torchwood back on the air, though that’s clearly proving difficult.
In the absence of a renewal for his own series, then, the best thing to do would be for him to return to Doctor Who for, say, a multi-episode arc, at least. The joy of Jack is that his charms mean he bounces well off any character and we’d love to see what sort of dynamic he would share with Jodie Whittaker’s Doctor. After all, Barrowman has been vocally supportive of the decision to cast the actress.