What began as a marketing slip, has now been confirmed by both Activision and Bungie as being official. The first expansion for Destiny 2 is named Curse of Osiris and will take Guardians to Mercury. From Bungie:
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
To us, it seems a little peculiar that we already know exactly what the first expansion for Destiny 2 is just over a week after release. That said, it seems to have a story component so we’ll be pleased if this first expansion is better than the initial expansion for Destiny 1 in The Dark Below. Time will tell!