Vex as the main threat after Final Shape? The clues are all there in-game right now, Bungie isn't done telling Vex stories, AND the Vex are in a "wait and see" position while the Traveler and Witness eliminate each other.
Destiny 2's story really improved this season. However, the piecemeal deliver of it is still a right pain and slows down all sense of tension and urgency.