Susan should return, played by Carole Ann Ford
Peter Capaldi has commented that the Doctor's grand-daughter Susan should feature. Of course there is a certain amount of popularity to it but with Capaldi referring to it that may make it more likely. Of course Carole Ann Ford is older than all the 8 surviving Doctor actors but this is science fiction so there is no problem. I would like Susan to occasionally feature, played by Carole Ann Ford, not just a few seconds of her appearing and then re-generating (if she is a Time Lady). In the audio series Susan's son and husband have been killed so maybe she then found a way back to Gallifrey prior to the Time War. Maybe she survived and can somehow send a signal for the Doctor to home in on and thus locate Gallifrey.