While The Darkness isn’t going away from the franchise entirely, it’s not being dealt with in Destiny 2. The game’s director Luke Smith says The Darkness was left out for a more focused story:
The Darkness was a vague term that wasn’t fully ever defined in Destiny 1: Was it meant to refer to all the enemies of the Guardians? Or was it a specific enemy race that was never actually seen in Destiny 1?
It was all left a bit unclear, although it appears that it definitely isn’t the former. Project lead Mark Noseworthy says the Cabal are not a part of the Darkness, which seems to squash the theory about some enemy alliance identified under that name:
With that said, Luke Smith says players are owed an answer to what the Darkness is, but that won’t be in Destiny 2. Instead, the main focus outside of Guardians trying to reclaim their Light is the villain of Dominus Ghaul. Players will learn more about Ghaul from cutscenes while playing the game, and discover how he unseated the Cabal Emperor and what is driving him to attack Earth and steal the Traveler.
And if there were any worries that some story threads may fade into the ether, like some of the narrative trails from Destiny 1, Smith says “[this story direction allows] Destiny 2 to usher in the rest of the saga. It’s going to start some threads that we intend to carry through.”
Destiny 2 will launch September 8, 2017 on PS4 and Xbox One, with a PC release coming at a later unannounced date.
Source: Game Informer