Many people are speculating that since each focus is tied to a specific element (arc, solar, and void), there will be a third focus for each class, as of yet unrevealed, which is tied to the only element each class currently does not have a focus for. So far, we've seen an arc and a void focus for the titan, a solar and an arc focus for the hunter, and a void and a solar focus for the warlock. According to this theory, the titan should have a third class tied to solar energy, the hunter should have one for the void, and the Warlock should have an arc focus. There's other evidence to back this up, but I can't remember it off the too if my head. Just a theory of course, but it makes a lot of sense. At least to me.