A prologue to series 9 has been released, showing the Doctor visit the Sisterhood of Karn once again. You can view the prologue here.
It raises far more questions than answers. The Doctor is hiding from an old enemy, though it's never revealed who that enemy is. You would assume it was the Master but given the fact they are described as 'he' makes you think it's unlikely to be the Missy-incarnation of the character. It's also suggested that the Doctor and this person have been enemies all their lives, which seemingly rules out virtually all other contenders- hardly any enemies have appeared more than a handful of times.
I have one theory on the subject- it's Death. It would make sense that the Doctor has been enemies with death all his life and if he's going into great peril it also makes sense the way the Doctor says "an enemy is just a friend you havn't got to know yet". It would also work with the gender problem, as Death is usually described as male.
If this is true, I doubt it means we will actually see a Grim Reaper-type figure on the screen- I suspect it's more the concept of death that the Doctor is fighting.
There is also going to be another prologue, a US exclusive it appears (until the box set presumably), entitled The Doctor's Meditation which looks to follow straight on from this prologue.
Also, I'm really pleased the BBC are finally calling these things prologues instead of prequels!