Bungie has said (I'm not the most reliable source in the world, but I know for a fact I heard this somewhere) that the vacant subclass spot was from an earlier feature where you could combine different areas of the subclass for your class for a perfect fit. But they took that out during alpha or som other time. Because, you know, it would be too damn overpowered. However I still believe it's only a matter of Tim before new subs arrive.