I like the Titan idea. Temperatures on Titan are around -179 Celsius, this would create a place where Bungie really has a truly amazing opportunity to do some great stuff. There are two main possibilities that I could think of if Titan was included in Destiny. The first possibility is an advanced form of life, far more advanced than anything we have seen, far more advanced than the Vex, Cabal, Fallen, & Hive. So advanced that they have built one large structure that houses all life, and the edge of it extends to outside of the atmosphere so ships can dock, and they can enter the structure. Or they are so advanced that they have created a way to change the atmosphere and environment so instead of -179 Celsius it is a nice 25-30 Celsius. The other possibility is that there is a creature that is so far out, it looks like it could not even exist. It could have a thick armor shell so it could survive the cold temperature and not be affected by the cold water. Or they have a thick coat of fur that would allow them to survive the cold temperature.