I can’t tell who the old familiar face might be. But there were some suggestions like Jefferson or Elsa. Probably not likely, but maybe. Amy (Merida) seems to have hinted that maybe she might show up in s6. Maybe it is Dorothy, since she was hinted to be filming in the season finale.
It’s definately possibly and most likely that Maleficent, Lily, Tink, Nova, the fairies, Red, Mulan, Dorothy, Aurora, Phillip, Dr. Whale, August, Gepetto, Jiminy, Ariel, Eric, Blue, the dwarfs, Granny, Abigail, Fredrick, Hansel & Gretal and their dad, Cinderella, Thomas, Maybe Jefferson and Grace, Sir Maurice, Lancelot, Will Scarlet, Anton, Pongo, Toto, and maybe Elsa, Anna, Kristoff and Merida might return in s6. Probably some of the still living villains will return. Like that jerky brainless oaf Isaac Heller (since he was arrested for good) the Magic Mirror, and King George. They are still alive and around. I can dream.
Those New York scenes to them returning to Storybrooke all seem to take place in Only You.