In the debut trailer for the game, the narration explains that the Old Ones built towers that reached the sky. This might be an exaggeration, or it might not. Amongst some of the text files that players can find scattered about the world, several mention a project called "Far Zenith." Much like the Zero Dawn project, it was an attempt to preserve humanity in the face of its imminent destruction by the Faro Swarm. Instead of going underground though, the idea was to use a spaceship called the Odyssey to relocate to a neighboring star system. Sadly, it was a total failure due to an matter/antimatter power generator accident while exiting the solar system. That might make it sound like a dead end story-wise, but in order to build something like the Odyssey, I think there would have to be pretty extensive infrastructure setup in orbit around earth. Things like an space elevator connected to a ring habitat and geosynchronous assembly yard would definitely be within the realm of possibility. Here's the kicker though, "why would anyone go up there?" As it turns out there could be a very good reason to do so.
Story aside, I think there's also some room for technical improvements. For one the water physics could use another pass, particularly with regards to how it interacts with things moving into, through, and out of it. Lip Syncing and facial animations (while not as bad as Mass Effect: Andromeda) could stand some improvement, as could the way robotic herds operate. Instead of remaining in at set zone, think it would be a lot more interesting if they migrated across large portions of the map. This might make it a bit harder for the player to find particular types of robotic animals, but I think the problem could be circumvented by having hacked Tallnecks constantly update the player via moving map icons. I also have this idea of a nomadic desert people who shepard robotic camel/turtle robots that have a big tank (or two) of fresh water on their backs. Being able to ride a Sawtooth would be cool, or even more awesome - a Stormbird. Fighting against ape-like robots in the jungle or forest might be fun as well. Although that last suggestion might be a bit too on the nose given the name of the developer that made Horizon: Zero Dawn. Regardless, I look forward to seeing what the come up with next.