I am actually somewhat underwhelmed by the "open world" aspect. It seems rather deserted and desolate. I understand the whole of humanity is hiding in the city and only the guardians have started to really venture outward, but where are the lives and communities built by the enemies? All you've seen so far are relatively small squads of enemies. The Earth currently has what, 6 or 7 billion people? In a "golden age", I'd expect that population to at least double as people have 3x lifespans and can migrate to different planets. With that in mind, an enemy force that can so completely overwhelm those numbers should themselves be comparable in size. So where did they all go after the Collapse? According to the story, it's been several centuries since. Have they all just been sitting on the sidelines, twiddling their thumbs? Shouldn't there be continent spanning crypts filled with millions of Hive Thralls, Knights, Acolytes, etc? Even if the Fallen have "fallen" I'd expect their numbers to still be overwhelming, when faced against a fireteam of 3 guardians. The amount of force it took to essentially "take down" the House of Devils was rather laughable, in the scope of things. Where is the "oh shit" moment when you stumble into a cavern full of thousands of waking Hive? Or the ruins of a city crawling with hundreds of Fallen troops, comprised of several houses? While the general scenery did convey as sense of loss, I felt there lacked a sense of tension that should exist in a world lost and overrun by dark forces that could at any moment, rise up and snuff out the last of humanity.