Maybe this has already been thought of or speculated about but there is a sort of direct link between the factions; to include the Vanguard, and the enemies of the city. Here is my attempt to relay something that I've been thinking about since year 1- Future War Cult/The Vex- Obsessions with manipulating and experimenting with time. The Mysterious Stranger Exo has links to both the Vex, and FWC. Dead Orbit/ Fallen- Post traveler fleets would eventually run out of resources that they bring with them, and would eventually take to piracy and scavenging. New Monarchy/ The Cabal- This one is a bit more of a stretch, and really only has to do with the Monarchy portion of how they operate. We know that the Cabal are part of an empire, and extremely rigid in how they operate. Vanguard & Risen/ Hive & Taken- Both manifestations of the Darkness and Light, using the subordinate beings to meet the ends of there will. I would like to hear if anyone else can elaborate on the links.