Actually it's even more interesting than that. The cabal we meet and fight aren't expanding, they're *running*. Don't remember if it's exactly confirmed but I'm pretty sure the cabal we meet are literally the last of them.
One thing that I think Bungie does better than almost anyone is their alien stuff. Each of these races is totally unique in design and background. Unfortunately the game raised nothing but questions about all of them while answering virtually none of them. Though The Hive seems to have gotten more attention overall than the others.