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.
I'm another one on board with "He's whoever you see him as", and I personally see him as an alternate 4th that grew to that old age.
as to "revisiting a few old faces, but, only the favorites", that sings out to me a wink to the fans who rewatch their favorite Doctor Who episodes/Doctors and the statement also has the advantage of being able to be taken in a myriad of ways to support your own personal Canon as to who the Curator is.
One Day I hope to be the Man my Cat thinks I am
Where are we going? And why are we in this Handbasket?