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.

Just wanna say your username is great.

Awesome new information from the Destiny Companion seems to suggest that the trio does not stop with classes alone. I cannot wait to get a chance to scour the program. We still have nothing secured until the game launches, but my dreams of having a third flavor of Titan is sailing on Solar winds right now... =)

I'm gonna say it, I think Byf is wrong, there I said it. I was listening to that part of the podcast a few minutes ago and shaking my head. I'm happy with 2 in fact I'm not even surprised with just 2. Look at how balanced everyone said the multiplayer is, with supers and there various iterations and 2 vary different focuses already in the game, I feel confident that this is how the game will ship. If as was said the 3rd element means a third focus I would bet it is introduced in the first "Comet" DLC as a way to put new life into even your most established characters. I have more to say but I think I'll stop there

Here is what I want for the last three subclasses... (speculation) Titan Solar: Erupts into flames and grants a speed boost. Spurting out solar flames to do damage over time to all foes near him. Void Hunter: Becomes invisible for a set time, and when reactivated or time runs out, bursts out a void black hole that destroys enemies into oblivion. While invisible, you are allowed to blink a couple of times, but they are larger blinks then the warlocks. Arc Warlock: Something like Kennen from LoL where he turns into a cloud of arc which can move through walls and disintegrates foes.

Notice: There are three sources of 'energy' for our Space Magic. Arc Light, Void Light, and Solar Light. The initial offerings of subclass only cover two of those energy sources, which likely means; at the very least, a third subclass of each of them. Possibly an end game subclass may exist incorporating all three energy sources into one synergistic subclass of devastation (Never cross the streams!). [EDIT: They are all called Light instead.]

I would definitely be willing to believe bungie is going to hold onto some stuff until launch, they are pretty good at keeping secrets, entirely a possibility!

Bungie has indeed eluded to before that besides the two subclasses per class we seen so far, other sub classes can be found in game and "equipped" like gear.