Re: The Curator.... Who is he? (SPOILERS for "The Day of the Doctor")
I think Baker was playing a guy who just happened to look like an older version of the Fourth Doctor, who was replying to some young guy's comments on "retiring" and becoming a "great curator". I don't think Baker was playing a future Doctor at all. I think it was just a wink at the fans. I'd be surprised if anything more comes of it.
I don't think he was playing a specific incarnation or that there is any story to be told there and none ever will be. I personally like to think that he is the man the Doctor becomes when he has saved everyone he can save, seen everything he can see; the man he ends up as when his time-stream has become so knotted after innumerable centuries in the TARDIS that he finds one of the few points in reality in which he can exist without imploding the whole wibbly-wobbly mess and finally retires to rest his old bones in a favorite time on a favorite planet.
"There is no reason why good cannot triumph as often as evil. The triumph of anything is a matter of organization. If there are such things as angels, I hope that they are organized along the lines of the Maffia." - Winston Niles Rumfoord.