In the Alpha / Beta the subclasses we have now had a 2nd page. It was never unlocked during beta but it read something like this. You know how we select whatever perks. Well after maxing you could choose to lock all of your choices in and then move onto the 2nd page of skills. Something else that might of changed between release or was simply a placeholder, but I definitely remember seeing it. Also I think its pretty clear all classes will get a 3 subclass eventually. To fill in the holes for each element. Warlock - Arc Titan - Solar Hunter - Void