Destiny 2: Curse of Osiris got a new trailer and a release date at Paris Games Week 2017. The new DLC will bring a whole new chapter to the Destiny canon, and open Mercury as a visitable planet.
An ancient gate has opened on Mercury and Vex from the past, and the future are invading. Osirus is a controversial guardian has returned to help with the fight, but he may be as big a threat as the Vex. According to Destiny lore, Mercury used to be a garden world in the Golden Age of Man, but the Vex transformed it into a machine world during the Collapse. Apparently, between then and now the Vex turned the planet into some sort of time portal. There were allusions to the Vex having some kind of time manipulation technology when you looked at certain scannable in the base Destiny 2 game, and now we're getting to see it front row and center.
There will also be need raid content added to Destiny 2 with the release of Curse of Osiris. Whether that means it will expand the current Leviathan Raid (which wouldn't make much sense), or if there'll be a whole new raid entirely. There should be some new strikes and PvP maps thrown in for good measure too.
Destiny 2 Expansion I: Curse of Osiris releases on December 5, 2017.