Ok, I can see a lot of the stuff you've got does make sense. I've had a running theory that I've been working on for a while.
Starting with Cabal, a lot like you said, an imperialistic group of brutes that conquer or destroy other races, as shown by the Psions being a probable different species than the main Cabal forces. Now, while they have been conquering worlds, they still haven't exactly been left untouched. In the Grimoire, theres a ghost fragment that says the ghost overheard them talking about fleeing something. This could possibly be the Darkness. So they may also have lost their homeworld, and are trying to make up for it by creating a massive empire. Or they may be trying to protect their home by creating a huge buffer zone of planets that they can use to defend themselves and keep their homeworld safe for a while at least.
The Fallen are the most interesting race to me. I think that they seem to be a race that was once actually a lot like mankind, and had been found by the Traveler. After they made contact with the Traveler, they reached an enlightened era that was the equivalent of the human golden age, which is why when you shoot them in the head, light spills out of their necks. They are also terrified of the Hive, who seem to be the Darkness' servants, and model their servitors after the Traveler in a sense. However, some sort of social or cultural upheaval happened, and they may have actually obliterated their own planet and destroyed their social structure, reducing them to marauding pirates and scavengers, who then follow in the Traveler's wake in order to try and gain its favor back, but costing them even more of their advancement and driving them further into their marauder lifestyle.
The Hive are the servants of the Darkness. They seem to be polytheistic, with their gods "living" among them and guiding their society in between periods of dormancy. However, they have a god above all gods, which is the Darkness. The Darkness controls them and instructs them on what to do, which is why they hunt the Traveler and seek to extinguish light. They also might be able to corrupt other species, leading to the diversity of their forms, a lot like the zerg from StarCraft. But this worship of the darkness is why they have that growth that spreads wisps of darkness and why they worked to corrupt the Traveler using its fragment in one of the missions.
The Vex are, and pardon the pun here but it seemed fitting, vexing to me. They seem to be incredible logical and trying to accomplish that great goal of all interstellar hyper intelligent robot races and wipe out all sentient life in the galaxy because of their irrational behavior and damaging effects on their surroundings. However, they also seem to have some other motives that revolve around whatever it is in the Garden. If I remember correctly, somewhere it says that they may have actually created that entity, either intentionally or through some accident. Either way, they didnt understand it, and couldn't comprehend its power, so they came to one logical conclusion: to worship it in order to attain its favor and power. My best guess is that whatever it is, it may be a fragment of the Darkness taking a physical form due to something the Vex did, thus creating a separate entity from the Darkness that the Hive worship. This entity drives them forward, much like the Hive, and pushes them towards galactic conquest and some hidden motive.
These are my theories on the races at least.
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[quote]and model their servitors after the Traveler in a sense. [/quote] This is very interesting.
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Also, the name 'fallen'... Fallen from grace... Fell out of the travellers favour. I like this idea a lot +1