Re: The Curator.... Who is he? (SPOILERS for "The Day of the Doctor")
I'll go with him being the 4th Doctor aged up due to shorting out the time differential (see "Time Crash").
Smith: I could be a curator. I'ld be great at curating. I'ld be... The Great Curator. Ha ha, I could retire and do that. I could retire and be the curator of this place.
Baker: You know, I really think you might.
Smith, staring in astonishment: I never forget a face.
Baker: I know you don't. And in years to come you might find yourself revisiting a few, but just the old favorites.
Read what you will into those lines, I took it to mean a future reincarnation of The Doctor, beyond the 13th. A Doctor who's retired and taken up curating.
To me "And in years to come you might find yourself revisiting a few" comes off more as a reference to him crossing his own time stream, which he has a tendency to do, rather than him reusing old faces. The "but just the old favorites." crack was a rather cheeky fourth wall joke alluding to the fact only fan favorites Tom Baker (4th) and David Tennant (11th) reprised there roles for the special.
"I calculated the odds of this succeeding versus the odds I was doing something incredibly stupid, and I went ahead anyway." - Crow T. Robot, Mystery Science Theater 3000: The Movie