With the Doctor joining Clara inside his time stream, we see that she's fallen into a stormy, foggy, echo-y place and is lost. Everything around her is the Doctor, but isn't. She can hear his voice and see his previous forms, but not him, not his 11th face. So he sends her something, not from his past but from hers. It's a leaf, the one that began her life, floating down to her. Hold tight to it, he tells her. It will take you home.
But also: The Doctor arrives! (Moffat, preserve your climaxes.) She's wary but he urges her over. How many times now has she saved him? Let him once return the favor. They embrace and are assuredly on their way to saving each other when the Doctor sees a figure, out over on the edge, his back to us. Who is that? Clara asks. It's me, the Doctor says, but with a different name -- not like the Doctor, who chose his present title. Names are like promises, and this man is the one who broke the promise. Clara collapses into the Doctor's arms, dizzy from the convolutions. "He is my secret," the Doctor tells no one/us.
The man speaks: "What I did, I did without choice in the name of peace and sanity."
The Doctor replies: "But not in the name of the Doctor."
The man turns: Introducing JOHN HURT as THE DOCTOR
That's right, y'all: a development so enticingly arbitrary that it probably allows any possibility. The final bit of the finale is almost certainly a toss to the 50th Anniversary special in November, also a Moffat joint, of which Hurt will almost certainly play a role. Setting aside the tasty bits from the rest of the episode -- sketching in more moods for the Doctor, post-Amy/Rory and post-River, other than "inquisitive melancholy" -- this is Moffat's big, mythos-expanding stroke, and an anarchic bit of wordplay besides. (I'll give you all of the names!) I thought the episode was really strong before the reveal, but I'm not sure it's any better because of it. Do you like the direction Who is headed? How great/sad (or sad/great) was River's return? And isn't it possible that the Doctor's name is John, or am I just being a lousy American?
All speculation is welcome below, as are all revisions, retconning, reduxes and un-duxes. Timelines be damned!