The description also confirms that you'll learn more about the mysterious Osiris - who we've previously heard about but never met:
"Destiny 2 Expansion I: Curse of Osiris continues your Guardian’s journey with all-new story missions and adventures set in a new destination, Mercury. Journey through time and space to learn the secrets of Osiris, avert a dark future, and rebuild the ties between the legendary Warlock and his greatest student - Ikora Rey."
All of this correlates with a previous source - the only major detail missing here is mention of the first game's Lighthouse location becoming a new social hub. Given how much was correct in the original leak, we'd expect that be true too.
source:http://www.ign.com/articles/2017/09/08/xbox-store-confirms-destiny-2-dlc-leak