I recall seeing actual novels set in the universe. I wonder if any of them could be worked into a game storyline.
Funny you say that, as Reach is based off the novel that preluded the first game, though it's from a very different perspective...
It was like a biography, really, following John, or 'Master Chief', from before he was even a sperm cell (yes, they had plans for him before he was even technically in existence) right up to the opening of the first Halo game and the discovery of one of the Halo instillations.
In that period, we see his childhood, his kidnapping, bonding with his few hundred fellow super children, rising to be their leader, and when they get old enough, they undergo some pretty radical enhancements, put on so many drugs to induce growth, even going as far as to put armoured plating on their bones (I think, at least... grafting of some kind was involved)
Only about 32 of them survived all of this, if I'm correct... insanely powerful bodies, perfect obedience, highly intelligent, nerves of steel... these guys were above and beyond anything else even before their Mjolnir armour came about. Once they started testing that, the Covenant showed up (Spartan's were originally to fight rebel's) and their shielding technology was then adapted.
The book ends with John fleeing Reach, under the assumption that he is the last survivor of the Spartans (later proven wrong) and carrying the data that the Covenant fought so hard for... he doesn't know what it is. No one really does. But Cortana, having intuition skills borderlining prophecy (that's kind of her thing, notice how much importance she puts into seemingly mundane choices, like choosing Noble 6? She's an accident of science, an A.I. so advance she can almost see the future) decides to get a Slipspace jump out of that data... and is lead to the first Halo.
Cue the beginning of the first game.