Doctor Who Series 10: 6 Theories On Who's In The Vault
2. The Master (John Simm)
Now we are getting into the less outlandish theories that are much safer bets. Come on, it just has to be some form of the Master in that vault, doesn't it?
John Simm's return to Doctor Who has been trumpeted for a while now, after the BBC decided to reveal his surprise appearance before the series had even begun. Given that, it is not too much of a stretch to suggest that it is he who is locked up in the vault. The vault is clearly covered in Gallifreyan symbols so it makes sense for a Time Lord to be in there.
More than that, John Simm's Master was last seen returning to the time-locked Gallifrey with the Time Lords at the end of 2010's The End of Time: Part Two. Now that the Doctor has discovered his home planet again - in 2015's Hell Bent - maybe they asked him to keep hold of the madman the last time he visited.
This would also sit with the fan theory that Missy is not in fact the Master at all and that she is really, I don't know, the Rani instead. Personally, I think it would be a cheap move to retcon Missy away like this but it isn't entirely out of the realms of possibility.