Destiny 1 will be the cabal emperor or another vex time Lord. The final boss ever should be the traveller as it is a vessel of the darkness, gardening world's so they are full of light for the darkness to feed from, we kill the traveller we kill the darkness' food, we kill the darkness. But we also lose our space magic in the process. The traveller sleeps because rasputin managed to shut it down/knock it out before it could flee the darkness and leave us as it did the fallen.