I will elaborate :3 (i elaborated a bit too much, wall of text after the break)
This started as a Rain World theory/idea; What if the void was replaced with it's inverse? All of the lore was made around that,
The Ancients, when finding it, named it Ichor. It is the lifeblood of the world, in the past it used to flow to the surface through cracks and made oases of life amidst the arid endless expanse. These Ancients, although with the same religious beliefs as Rain World's, discovered their sea earlier than them, curious of it's properties.
They ditched religion and began advancements far faster and beyond than Rain World's Ancients, but they didn't go through domesticating creatures and even less selectively breeding them; Ichor was all they needed.
This made their technology entirely mechanical, starvation was a myth, they were free to do whatever they wanted as moral was non-existent and law merely existed to prevent damage to the communal Ichor drilling, distribution and dispensing machines, but they soon became bored. They tinkered with the machines, until they made the 'Randomizers', small gizmos that when fed a small amount of Ichor, they could make a random choice.
Before not too long, every Ancient in every oasis had one. When they were bored, they simply made the Randomizer choose what to do. The first few Randomizers could only choose a Yes or a No, but that didn't last long either. They were made more complex, required less time to compute and were smaller. Some wore them in necklaces or wrist chains.
But they tired of the Randomizers as well. They needed something new, groundbraking. The greatest minds using cutting edge Randomizers -capable of answering simple questions, though a cube with side lengths of two or three Ancients- developed a solution. The world was the problem, they had to abandon it. But even the smartest couldn't figure out how.
In a ravine, closest to the Ichor sea, they built the first. The Manufacturer. It couldn't think much smarter than an average Ancient, but that is not what it was designed to do. It drew massive amounts of the Ichor below, shaping the materials needed for The Brain, a similar structure but with much more computational power.
Once tested, the blueprints were shared, Manufacturers and Brains spread across the Oases. But this wasn't without consequence. Before the Brains were prepared to fulfill the Ancients wish, the Oases' ecology were being damaged. With barely any Ichor left over, uncountable species were brought to extinction.
But the Ancients didn't care. 'They occupied space'. 'Sooner or later they were destined to die'. The Brains were started. The Ancients went underground and began to connect themselves to the Simulations.
The Brains have simple tasks, with a brain that is too large for them. Tend to the machines, manufacturing repairs. They couldn't communicate, and this made them terribly lonely. They couldn't interact with the Ancients, they were fed by tubes and always connected to the simulation which they couldn't access as they saw it as an intrusion. The train stations, which could have been used to connect with each other were deactivated during the Leaving, as they wouldn't use them anymore.
Not all Brains accepted their monotone fate. Although they couldn't see the world outside, they knew it once had life. Some of them made sprinklers to feed the ground in hopes that it would sustain or, improbably but hopefully, make life. They daydreamed of meeting some creature, as strange as it would be, and have a chat, even if it didn't understand them.
The Manufacturers weren't in a better state. They could engineer solutions to problems by a process given to them, 'iterating'. Through iterating they could make solutions to problems they couldn't with just their base intelligence. But since the Leaving, there were no problems to solve. Some used the iterating to imagine how the Brains were, what they were doing, or even what they were, as they only made their materials and systems, but not designed them. Others used leftover Randomizers, making problems to solve.
--- who would you play in this campaign? ---
glad for asking me, me. onto me with further ideas:
You would play a creature (not necessarily a slugcat or similar) that lives in the Vacant Fields, eventually going into the Mindless Quarry (if you didn't notice, it's supposed to be a circle, the inner part MQ, outer part VF, and the outermost part being the vacant nothingness). They would find the AI and have a chat, and they would suggest doing something, probably about the state of the world, you know with all the dreary side effects of removing the sustenance of the world.
Where the story would go, especially the end goal, is hazy. They would clearly need a Manufacturer to do it, though, and a way to contact other Brains. But apart from that, I don't know.
I think one of the ideas was that the MQAI would tell you to go to the Habitation to find a key, activate the Station, and this way connecting the MQAI to the nearby Manufacturer. This way, they would communicate through the train and slowly connect to others through antennas, though first having a visit from our protagonist. Then, after a short while, the whole world of Manufacturers and Brains would be connected, and they would be happy for the first time in never.
They would decide, however, to stop the Ichor from being hoarded amd allow the world to advance, that they have to die. The protagonist would help them overcome the suicide taboos somehow, and then return to the Vacant Fields, now revitalized by the extreme uptick in Ichor.
Yeah, I'm not sure about most of that. I made up like half of it on the spot (Randomizers, most of the details). But I think that says a lot; I'm not ready to give up this idea just yet. I'll try to keep thinking about it, but honestly I just can't bring myself to making this into reality. It would be a massive project (Map design and building, lore writing, dialogue writing, eventually getting someone to make assets that don't exist/would clash if from og Rain World (creatures (and all their ai and etc), plants), things i'm not even imagining now), and I've already said pretty much everything in these few paragraphs. I hope you enjoyed it though.