I don’t have a full sci fi save. My save is a fantasy save with some sci-fi elements. I have a Sixam Alien outpost parked in Strangerville and another one in Oasis Springs along with a training camp for alien hunters. There was also a Men in Black type unit operating out of a high tech warehouse in Willow Creek. I say “was” because they were sacrificed when my save got corrupted and had to be migrated. I also have the 10th Doctor parked in HoB. All of my aliens, except the Doctor, are trying to fly under the radar, so their homes fit in with their surroundings but are high tech/ futuristic on the inside. Although, The Doctor does have a quaint little cottage in HoB with the Tardis parked out back.
Basically, my game is separated into the human sim world and a magical realm called the Myst. Everything is set in modern times. I group game worlds into one of these two larger worlds and use mods to control who can travel where. I think you could do something similar with your game and have a futuristic type world alongside a less developed world or even a post apocalyptic, Mad Max type world. Group the game worlds where they are best suited and work out their relationship to each other. In my game, The Myst was created as a refuge when magical creatures got tired of being hunted and persecuted by humans, so there’s an adversarial relationship between the two lands.
Anyway, I think your idea is doable. I hope you’ll share more when you get it done.