This reminds me of an old RPG called LotR: The Third Age where, after completing each chapter, you could relive it but on the Dark Side. It was fun gettin' rid of your heroes, hahaha.
In Destiny, we could have some sort of "Villain Mode" where we could play as our foes. Or maybe make it part of a special room in PvP.
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I dont want to play as villains... my idea has nothing to do with them being villains.
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I don't see them being playable anytime soon, at least not in the way you ask for. With the information we have at the moment, the Fallen don't seem to have any redeeming qualities. That means we're just purely speculating.