Re: The Curator.... Who is he? (SPOILERS for "The Day of the Doctor")
I think you'd have to be intentionally obtuse not to see the obvious: the Curator is the final Doctor. There's really no ambiguity. He gives Eleven(Twelve) hope that Trenzalore isn't as fixed a final resting place as it might appear.
Some say that his arrival was foretold by a Check Engine light shining over Bethlehem and that he was born in a manger on Christmas Day.
All we know is, he's The Baby Stig.