Doctor Who Series 10: 6 Theories On Who's In The Vault
3. Susan
If it's not the Doctor's wife in the vault, then how about his granddaughter? Ordinarily, we wouldn't put much stock in this theory, but Moffat has shown a willingness to indulge the fans this season more than ever before. e.g. John Simm's Master, Ice Warriors, Mondasian Cybermen.
Speaking of the Mondasian Cybermen, Susan's return would continue the throwback feel to the First Doctor's era. More than that, actress Carole Ann Ford recently visited the TARDIS set and met Peter Capaldi. The uber-fan that he is, Capaldi repeatedly said that he felt Susan needed to come back to the show. Did Moffat grant his wish?
The biggest piece of evidence, though, has to be the rumour that the First Doctor himself will appear in the upcoming Christmas special, as played by David Bradley (who portrayed William Hartnell in 2013 biopic An Adventure in Space and Time). If the First Doctor is returning, why not bring Susan back too?
We're not sure how this would match up, but there is also another theory growing that Bill is actually a regenerated version of Susan. Evidence includes Bill nicknaming him "grandfather" just as Susan used to and their general familial relationship. I'll leave that for you to ponder.