This is the one we know for sure. At a Summer TCA press tour panel, Sarah Paulson confirmed Jessica Lange's return to the series and even nailed down a character and episode. "I do have the great pleasure of letting you all know that, yes, she will be back. Jessica will be returning as Constance in an episode I'm going to direct." The Wrap added that Lange is set to guest star in episode six.
Constance's return is perhaps the most obvious; not only does she survive until the end of Murder House — she's one of the very few survivors, in fact — but she also has assumed a role as Michael Langdon's caretaker. Yes, Michael Langdon aka the Antichrist, who has been confirmed as a returning character.