River Song
For most of these characters, we have to argue our case as to why now is the best time to bring them back. For River Song, however, it would be a crime not to bring her back. She’s the Doctor’s wife and (intimately) knows each and every one of his different incarnations. Of course, she has to meet the Thirteenth Doctor!
First appearing back in 2008, Alex Kingston initially turned up as the time-travelling archaeologist opposite David Tennant’s Time Lord before becoming a regular alongside Matt Smith. Back in 2014, though, she also encountered Peter Capaldi’s regeneration. That episode documented the fabled last time the lovers would meet, so in effect it brought River’s story to a close. While that was her last appearance on TV to date, however, the character has featured in several official audio productions since – pairing her up with a number of past Doctors – so it’s clear that there’s a lot more room to explore here.
Most of all, though, we’d just like to see River’s reaction to the Doctor becoming a woman. She adores all of the Doctors, but this is a bigger change than most, so it would be fascinating to see how the very different dynamic would be explored.