Location
Does your story need to start from Cheyenne Mountain and center on the western United States?
Nope.
Skyline modules are written with that location in mind, but only because it aligns well with the story from Horizon itself. If you want to run your game out of Madagascar, or Machu Picchu, or Manchester, or wherever, that’s fine. Anywhere on Earth is plausible within what we know about the world of Horizon.
We know that ELEUTHIA-9 was never intended to be the only Cradle facility. There are more around the world — as many as could be built without revealing their locations to the war machines. For example, ELEUTHIA-01 was referenced to be in Xinjiang (northwest) China, sealed just before the machines overran the area, while ELEUTHIA-02 was planned for Mt. Namuli in Mozambique. Even the locations which canonically fell to the war machines first are fair game — it’s reasonable that an ELEUTHIA Cradle could have been built beneath a devastated area after the machines had moved on. The holographic introduction to GAIA given by Elisabet Sobeck shows locations of planned Cradles around the globe.
It’s a fair assumption that, for simplicity and because engineers dislike designing things more than once, most of the Cradles around the world likely used similar building plans, or at least similar architectural components. The Cradles could be assumed to be roughly equivalent in size, leading to roughly equivalent capacity, roughly equivalent food stores, and therefore roughly equivalent dates when their charges would need to be released to the surface. This would lead to any other peoples encountered from other Cradles having similar levels of technological advancement, similar gaps in their understanding of history, and so on.
It might also be interesting to meet people whose ancestors had been released a century before or after the Nora. Similarly, to meet people whose ancestors had access to more written history than the Nora, maybe by an Old One who had left a cache of books, or tablets with a backup of the Wikipedia database on them. That might give such people an advantage on understanding technology, but it would also likely burden them with unresolved arguments and biases of the Old Ones.
Newer & Sealed Cradles
It’s also reasonable that new ELEUTHIA Cradles could have been built by AIs after the war machines had gone dormant. Something akin to a Stormbird or Shell-Walker convoy could transport cloned gametes literally anywhere, to some Cradle carved out of the Earth, using the same excavation system which created the Cauldrons.
Those later Cradles might also be narratively interesting — if they were built after DEMETER’s terraforming started to take hold and produce food, which could then be ferried to the Cradle by machines, those Cradles may still be populated with small groups of people who have never seen the outside world. Such groups might then have a better understanding of the Old Ones and their technology, assuming functioning servitors, but might know nothing of the machines or peoples outside.
Similarly, if other Cradle facilities did not run into the same problems as ELEUTHIA-9, they might have radically different release dates and experiences, regardless of the age of the Cradle.
Lone Survivors
It might be tempting to avoid all the complexity of figuring out what people from another Cradle would be like, and instead decide that ELEUTHIA-9 was the only surviving Cradle. Such a world might be appropriate in a post-apocalyptic horror campaign, where the farther you get from Nora lands the fewer people you run into, until you go weeks of travel without seeing another human.
At a walking pace of 25 miles per day, the 3600mi travel just to the Panama Canal would take almost 6 months. Getting down to Cape Horn from there would be another 8 or 9 months on top of that. While it’s possible that human settlements have made it that far in the 500 years since the opening of ELEUTHIA-9, it’s unlikely.
Alternatively, you might decide that humans were more easily able to travel and relocate safely before the Derangement. Gathering resources to create settlements might have also been easier, leading to several hundred years of rapid expansion across North America. Ex-Europeans made the same journey across the continent in just under 300 years (circa 1500 to 1800 CE), yielding settlements on the US West Coast within a few decades after that. It’s plausible, especially without the threat of carnivorous predators, that the ELEUTHIA-9 group expanded to cover effectively the same areas of North America that the Old Ones inhabited.
If the one surviving Cradle facility was ELEUTHIA-01, in northwest China, there would be an even larger area to explore.
Fauna
Canonically, the world of Horizon does not include megafauna. For details on the animals of the world, see Animals.
It would be plausible, however, to amend that to say that there are no megafauna in areas in which larger animals would compete with humans. That is, maybe horses have been reintroduced, but on the other side of North America, or on another continent altogether.
It would take time for any megafauna species to establish itself in the ecosystem. This might have led to regional restrictions if ARTEMIS believed the megafauna species would not have a chance to grow naturally due to hunting or other concerns. Species receptive to domestication and labor would be especially hunted — the entirety of a horse population, for example, would likely be hunted to the extreme once humans figured out they could be used for labor.