On the other Xen thread, we talked about how it should begin as an alone, and desolate wasteland, but should evolve into a more NPC-heavy and scripted world where, in some way or another, Freeman participates in the Vortigaunt uprising.
There’s a bunch of points supporting and opposing the idea, but I don’t feel like dumping em all here, but I think implementing things like that, or other removed portions Xen, would be an interesting way to expand the portion, without deviating too much.
Also, though, these are probably obselete discussions. We have no idea how for along the devs already are, and the things they’ve already implemented. All these threads with all these ideas are probably junk to them at this point, because it’s not like they’re starting from scratch. They’re just opening a few windows for work.