So with the release of Destiny 2 foretold in 2017, these are three of the ways I think it could open. 1. You remember the Psion Flayers? Well they pulled A MOON FROM ORBIT AND KEPT IT THERE. We killed 3 flayers but Phobos has not crashed. If there are more flayers, they can be ordered by the cabal to crash it into mars, or maybe crash Luna into Earth. Alternatively, the psions could rebel against the cabal, starting a civil war that could destroy a lot of our solar system. 2. Rasputin does not like the Traveller. If you read the grimoire on him, you discover he was the one who controlled SIVA and let it loose on the iron lords, purely because they were guardians who entered Site 6. We also know that 'he doesn' t usually welcome visitors', and the last team before the Fallen S.A.B.E.R strike were brutally killed by him. Rasputin could decide to destroy a lot of the tower before we could even react. At this point Rasputin is a free agent. He was the one to cripple the Traveller, and he could do it again. 3. The Wrath of The Sky. This one is pretty far fetched but hear me out. So we know that Rasputin crippled the Traveller to prevent it from leaving, but does the Traveller know that? For all it knows, is it came to Earth, the darkness found it and it couldn't run. It can't even trust its own memories anymore. The Traveller has been shown to try and wipe out entire species just to stop the hive being born, and if it thinks we deliberately kept it trapped on Earth, this would be a fight we couldn't win. It could destroy our ghosts, steal our light, or maybe even go back to the Fallen. Anyway, these are my theories, let me know yours.