By: +David Herron; Date: May 3, 2017
My theory is that the John Simms Master is in the Vault.
The Doctor has in the past been charged with dealing with The Master, notably in the 1995 TV Movie. In that show, the Master was trapped in a container, and then managed to escape, taking over someones body, and nearly killing the Doctor. The Master is certainly a dangerous enough creature to keep locked up, and one can imagine the Doctor having responsibility for keeping The Master incarcerated.
Omega in the Vault? Nope.
New Ice Warrior? There's an Ice Warrior to appear later in the season. Doesn't sound likely. Why not a Cyberman, since Cybermen of all stripes are due to show up in the season?
Rassilon? Valyard? Rassilon appears to be wrong as well -- Omega and Rassilon are too powerful for the Doctor to be involved with entrapping those people. The Valyard is a different story, as in the Valyard is simply the Doctor. It's completely unclear how the Valyard will fit into the Doctor's time stream, however.
The Valyard was supposed to be the 13th Doctor. Actually, the Master described the Valyard as an amalgam of the Doctors darkest parts, dated somewhere between The Doctor's 12th and final incarnation. Now that the The Doctor's lifestream is coming around to Doctor #13, the Valyard must be addressed.
Missy is appearing at the end of the season, and is a likely culprit for releasing what's in the Vault.
River Song? NO