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It's already known that Naughty Dog is working on another game in addition to The Last of Us Remastered and Uncharted 4: A Thief's End, and thanks to an update to the Linkedin profile of former Lead Character Artist Michael Knowland, which has been recently updated, we learn that said game is indeed an "AAA" experience.
http://www.dualshockers.com/2014/07/14/naughty-dogs-unannounced-game-is-aaa-departed-character-artist-worked-on-uncharted-4s-nathan-drake/