Having Dark subclasses would kill the game for me. We have been fighting the Darkness for years now, to join it now would be horrible for myself and many other players. In my mind, we would be turning from heroes into villains. As well, having a free for all player vs player vs enemy setup in all destinations would not likely be liked by players who do not necessarily like the crucible. I would hate to be doing something and be shot in the back by another player. If you wanted to do a player against everything environment, you could have an arena destination run by an NPC like Shax. Have it have exclusive rewards and secrets like the Dreaming City and be optional. If you wanted Dark Guardians, you could have them as NPCs that you encounter who perhaps hunt you and provide unique rewards for killing them. You could even learn pieces of their lore and how they fell to the Darkness. Perhaps as loot for killing them over and over, we could earn their armor as it was before their fall and their signature weapon. For their abilities, it would be a good way to test out the wilder supers with players to see what they think when they see it and then create a Light version of it. Like Warlocks with the ability to create concentrated hurricanes or Hunters with the ability to create hard light animals to aid them. You could name these Dark Guardians something like The Dregden or The Lightless.