Well, so in the Alpha there was enough space in the UI for at least three subclasses, both at character creation and in the subclass swapping screen. The subclasses were also heavily themed around Destiny's three elements, both in the color of their icons and their descriptions which went out of the way to describe them as Solar or Void or Arc. As we've only seen two subclasses per class, it seems reasonable to expect there to be a third to cover the element that the previous sub-classes haven't covered for that class. This means we'd expect a Titan who uses Solar Light, a Hunter who uses Void Light, and a Warlock who uses Arc Light.