Think about it!
The Vex=Death: The Vex are an inescapable force, immortal through time and a danger to all forms of life. They bring end to the living as if they were a key component of the nature of the universe itself.
The Fallen=Famine: The Fallen are a scavenging race, picking at the bones of the golden age. They are completely dependent on the machine gods that sustain them. All they do is consume.
The Cabal=War: The Cabal are a race of warriors, their very fundamentals based on the dominance of other intelligent species. A WARpath has been left in their wake that nothing will stop.
The Hive=Pestilence: The Hive exist as an aversion to natural order. They are a sickly force of mortality bent on a virus-like mechanism of sacrifice and mass production used to invade and hollow out worlds.
See! In all honesty, I feel like the Vex and the Hive could switch roles, but I only say that the hive represent pestilence because of their diseased appearance. Also, the enemy races heralded the dark age, did they not? Thoughts?
Edit: Regarding the conquest/pestilence debate, I used the pestilence interpretation because that's what I'm familiar with. In line with conquest:
Vex=Conquest
Cabal=War
Fallen=Famine
Hive=Death
English
#Destiny
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Makes sense in a way. Cabal are from Mars, Mars is the Roman god of you guessed it war, the hive are mobs of ritualistic monsters that use overwhelming numbers to overtake like locusts, the very opening scene your ghost revives you and leads you from the fallen who are scavenging earth which looks like no life is there, and the vex are time traveling machines using many more mythology in their arsenal gorgons, minotaurs, harpies all greek monsters.