What if its the master john simm’s master perhaps?

It has to do something with Susan. The episode had an unusual interest with her what with the picture being shown at specific points, and if memory serves at one point the Doctor says that he’ll be alerted to someone opening the vault through his Physic paper, meaning someone on the other side may have telepathic abilities, ie a Time Lord.

And the promise?
“One day I’ll come back, yes I will come back…”
So my theory is an older, maybe dying Susan is behind that Vault and somehow she’s connected to Bill, because why would the Doctor have such an interest in teaching her?

If my theory is right then it should be awesome finally seeing Susan again after all these years, if we’re lucky they may confirm that what she went through in the Big Finish 8th Doctor adventures is canon, would love that as they’re my favorite set of Doctor Who Audio Adventures.

The Master
Missy
Susan
Something to do with the Monks
Could be related to the Cybermen
Something related to Gallifrey

Perhaps it’s something new – I remember seeing this Gallifreyan folk tale (can’t find it now) about a man who slew death to avenge his lover, only to trade mortal wounds with the invisible creature. Maybe that’s what’s in the vault

I have a very solid theory on who’s inside the vault but I keep telling myself what if I’m right, then I will have spoiled the surprise for everyone…
But seriously…
Why is nobody suggesting the Valeyard? (Sorry, tried to resist!)

@Mithrandir
Oh that’s a brilliant one! Can’t believe I didn’t think about that even though I’ve wanted him in the modern series for ages!

And the latest episodes have kinda debunked my theory it’s Susan in the vault because the show is making out that whoever is inside is sinister.

Now that you’ve brought it up it fits In near perfectly!

I believed it was Missy for a while. But The Valeyard is a better theory.
Even better (this was not my idea) that its the Valeyard Played by Peter Capaldi and its NOT the 12th Doctor thats re-generating in the trailer but the Valeyard!

I’d really like it to be a surprise of real significance, as opposed to somebody like The Mistress or The Master (as much as I love that John Simm is coming back).

The Valeyard is an interesting idea. The Great Intelligence reaffirmed that The Valeyard will be one of the Doctor’s name’s one day and since I’ve read that The Valeyard comes from somewhere between the 12th incarnation (technically the 10th Doctor after his partial regeneration) and the end (which has greatly increased since Matt Smith’s regeneration), it’s possible we’ll see him again one day and I’d be interested to see that, but whether it’ll be today … I’m not so sure.

Missy/the master is the monks demon that leads Them and the monks are trying to get missy/the master out of the vault