This could mean that Ned Stark would return to bookend the show, albeit in a creepy headless skeleton form. Or possibly Catelyn Stark, which would also serve as a nod to the original books, in which Catelyn is resurrected as Lady Stoneheart -- a storyline cut from the series either for time or because it was very, very stupid.
Fans also speculated that Cersei's wine glass is poisoned, Jon Snow will realize that he's been diddling his aunt, and an exciting new character will command the Greyjoy fleet. Or Theon got a haircut. Or it's just an extra. Who knows? Another promotional tidbit adding to the furor: the recent cover of Entertainment Weekly, which eschewed the usual focus on attractive celebrities for a close-up of the wrinkled, scowling face of the Night King.