That's a very compelling idea, and terrifically expressed, but my two cents would be that "sinister" seems to connote a malicious intent rather than just a hostility toward the enemy. I'm on board with the cyber/exoskeleton premise, but rather than having been resurrected by humanity, maybe they could be a group that perceived of themselves as a next phase of human potential. In addition to "exoskeleton" the prefix also conjures thoughts of exobiology, or even genetic exons, so perhaps they experimented on themselves with the intent of surpassing the rest of humanity---or even dominating them---but when the Collapse came, they were nearly destroyed and were obligated to rejoin humanity in an alliance of convenience or else face destruction.  The "isolated hero" angle is great, but it kind of makes them almost too heroic---too easy to celebrate over the Humans. But if they were once sinisterly over-ambitious, there's a lot more potential for dramatic  conflict as the series unfolds. A lot more uncertainty that could contribute to some fantastic plot lines. It would also add a lot of depth in terms of whether some of them regret their ambition and some of them still hold on to it.  In any case, kudos to Bungie for getting this much speculation out of us with so few words.