Of course, that much could’ve been assumed as Lady Stoneheart (who is the post-Red Wedding resurrected and grotesque looking Catelyn Stark) had two characters important in the show in a pretty precarious position at the end of the last book.
Martin’s statement made it sound as though he has grand plans for Stoneheart, and that those plans will never be seen in the final season of Game of Thrones.
George R.R. Martin has mentioned before he wished Lady Stoneheart had been included in Game of Thrones, but the fact that he’s still writing a storyline around her in the sixth book for the A Song of Ice and Fire series is new.