The subclasses for all of the Guardians are based around a specific elemental damage. Striker is arc focused, Voidcaller is void focused, and the golden gun is solar focused. This leads me to believe that the third subclass for each Guardian will fill an element that their other subclasses haven't filled. For warlock it is arc, hunter is void, and titan is solar. I got this idea from the new Destiny video where Luke Smith talks about the raids (found here: http://destinynews.net/endgame-beginning/). Near the middle of the video he mentions the striker class for titans, but instead of saying striker he simply says shock titan. This leads me to believe that the subclasses are based on the different elements since he called it by its element and not its name. Also all of the other subclasses have a clear element attached to it that isnt duplicated by the other subclasses in that class. This just makes too much sense for this not to be true. Let me know what you guys think. I uncovered a prime piece of info, but you guys might have found something I didn't(or this was already known by everyone and im just restating the obvious). Edit: Fixed my mistake with the ward of dawn super actually being void damage and not solar.