Doctor Who: Every Version Of The Doctor Explained
12. Doctor Moon (First Appearance: 2008)
It's never been confirmed in the episodes themselves, but Colin Salmon's Doctor Moon from Silence in the Library/Forest of the Dead is actually a future incarnation of the Doctor.
A Doctor Who Magazine feature called Showrunner Showdown revealed that Steven Moffat (who created Doctor Moon) emailed Russell T Davies in 2008 to explain that, in his head, Doctor Moon was a representation of a future Doctor, who had died in River Song's arms.
According to Moffat, when River dies in Forest of the Dead and lives on in the Library's computer as a data ghost, she learns that that dying Doctor has popped his mind inside a moon to watch over her, worrying she'll get lonely.
Will we ever see Doctor Moon again? Even if not, it certainly changes your perception of his character.