G-Man remains a mystery still, but if VALVe is willing to start to portray the G-Man in anyway other than being a strange bystander who talks in an otherworldly accent. Now, if the proceed in this style, what could have happened is that he himself sent the Borealis to the cold and snowy...
Obviously, it is in developement. But, I think Alyx will take a grim turn after seeing her father die. She may be a depressed trainwreck, or she could become a vengeful demon.
My theory on this achievement varies with every post I read. With this one, if the wiki propoganda was true, then why were parts of the ship left behind? Was the ship teleported away in a hasty escape, or even attempted to be taken in to Xen?
My idea is that the ending of Portal 2 will end in a way that is hard to describe, or even to be understood. If it ends, or there are hints of "Leaking ceilings" and "Flooded testchambers." What if the Borealis is a huge ship... That ran tests on the Portal Device. After all, it is used for...
I think that there will be a Ratman lair with a poster of the Borealis or... Or maybe GLaDOS lets a little secret slip out during a temper tantrum. Food for thought.