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 he haunts the halls of the Top Gear production office and that, if you listen closely, you can hear his wails around Gambon.
All we know is that he's not The Stig, he's The Stig's drowned predecessor.