Doctor Who: Who is Missy?

It's being billed as a mystery - not just what the hell is going on with Doctor Who's dead-people-coming-back arc, not even what the grand plan is. But there's something else promised, most likely a cherry on the cake for fans. Because "Missy", is...?

"You know who I am..."

Well, everyone has their theories, and here are mine. One of them's probably true as they're a bit obvious, so if this spoils it for you, stop reading.

Missy is Clara!
Okay, this is a rather literal interpretation of the (possession?) scenes billed in the trailers. Of course Clara's good... isn't she? Whilst this is probably the safest option, I personally wouldn't want to see Clara's newfound personality explained away by having a dark secret.

Missy is the Doctor!
New Doctor, new bad guy, and they seem to be equals, so why not? It had precedent - an evil Doctor popped up in 1986, so maybe Missy is a side-effect of the new regeneration and linked to the new identity crisis. Is he a good man, or a good woman? A female future Doctor would be a good way to skirt around that issue. I wouldn't like to analyse her line about the Doctor being her boyfriend though!

Missy is the Master!
Which leads us onto this similar theory, only more out-and-out evil - the Doctor's arch nemesis, reborn. The last time we saw him, he was John Simm and probably trapped on Gallifrey. Now that's been saved, perhaps the Master had a new life cycle too? Would fans appreciate a bold new take on the character? (very bold, in places!)

Missy is The Rani! Or Romana! Or Susan!
Ones for the fans - the female Time Lords who haven't turned up yet in the series. Regenerating The Rani and Romana have been done recently in Big Finish, so anything's possible. But would they bring back characters last seen in the 80s?

Missy is the TARDIS!
No idea how that works, but it's appeared as a mad Victorian lady the other year.



Missy is River Song!
Oh come on, Alex Kingston only appeared last year.

Missy is somebody who was sneakily introduced only a few episodes ago!
You see that little girl who pops up in the middle of that episode you didn't like very much? That was her!!

Missy is God/Missy is the Devil!

Frankly, for Doctor Who, that would be a cop-out.