They could make their appearance a part of the dlc, where you meet one and are given the option to create a new character, but you go through an altered story (it could just be training that automatically ranks you up), then you can progress in the dlc as the new class. Or you could do dark versions of each class, which is a new enemy type, since the 4 enemy factions are getting pretty old. But make them playable and different, and a altered story depending on if you choose to serve the light or dark. So you can play as light or dark, and the two oppose each other in pvp, and it would be a good oppurtunity for pve events, such as the two racing to kill the same boss from different paths, or simply gaining points by killing a boss, and you get rewards for your faction winning.