My theory is respects to everything Destiny. Fallen - Are actually not so evil, I believe they had the traveller first, or the traveller offered them the same things we have but left them for dead when he realised they were too weak and that is why some grimoire mentions about the Fallen wanting to take the Traveller back from us. Which would explain the House of Kings obsession. Hive - The darkest existing enemy we know about, most likely the strongest and have the closest ties with the darkness itself. I think they along with the Vex will be the only true threat. Vex - There main goal is to raise Gods by undoing events, there's likely some Vex realm or home world where there's an enemy that links to the whole Vex system, we could potentially enter a future where the city is destroyed and Earth is seized by the Vex? Cabal - Are not evil at all, ive already gone in depth about this but I believe the Cabal's only goal is to try and escape the darkness, I don't think they have any beef with the Traveller or humanity and only invaded Mars as a last resort (w/ Phobos), certain grimoire discuss this and the coming of Oryx "taking" them in the new mission is pretty much the very definition of this (Oryx is the darkness coming for the Cabal).