Similar to how Halo2 brought in the Covenant as playable, could Destiny allow for a faction of Fallen to join the tower as Variks has at the Reef? They are the only other group that we know of who had the light before. If some DO start getting it back on Earth at the Tower in helping the Guardians, that would add a nice disrupt to the current status quo. This would be a fun way to open up the gameplay, and to also give the Fallen less of a pathetic hope to thinking they can get the light back, even make Guardians question the nature of the light/Traveler more.