D2 was riding on D1's success. What would D3 be riding on? Every question regarding the collapse has been answered sufficiently, with its villain dealt with and its main power source located and locked away, for what it's worth. Other than the Winnower whom has said in a ship's lore they'd see us soon and isn't really a villain more than the antithesis of creation, we're doing site clean-up for the aftermath of the Witness. Until the last of the Echoes is dealt with and possibly the miraculous healing of the Traveler's open wound, there is no D3.