My 2p worth of speculation:
John Hurt is the earlier version of the Dr, before Christopher Ecclestone after McCoy/McGann, the one who ended the time war/Dalek extermination, the “really bad thing” we are alluded to every so often.
PS I like River Song, hope she comes back as a ghost again, but didn’t her regenerating the Dr in the last series, Nazi Germany(?) reset the regeneration counter for him, using up all her own.