Update:
During a subsequent panel dedicated to the game, Naughty Dog revealed a few more details about The Lost Legacy. It started out as a DLC pack but evolved into what it calls a "story expansion" and a "standalone experience." You can expect it to be shorter than Uncharted 4, but longer than The Last of Us expansion Left Behind. What was shown during the reveal is the beginning of the game. The Lost Legacy is set entirely in India, though the city from the video isn't the only area you can expect to visit.
Sursa: Gamespot