For the end of the Destiny series I imagine there will have some sort of solar system invasion of a massive overwhelming force controlled by "the darkness" and all the gains you've made in Destiny to reclaim and vanquish the forces from Earth's colonies will be reversed as you go on missions to salvage and save what you can as the darkness closes in. Then as the darkness is about to descend on the last city of man you are on a final mission necessary to awaken the traveler. As you reach the end of the mission the traveler suddenly comes alive and emits a planet-wide glowing beam of light that eradicates the forces of the darkness descending on the last city and joins the final battle on your behalf.