I've recently joined this forum seeking opinion and expertise (loosely speaking) for concepts I'm considering using to write a story. It's a world-building thing. I've got no special name for this world, but it's supposed to be the OTL long in the future. The first idea I had is that civilization has naturally developed into an advanced ecumenopolis with space-faring and the cybernetics, but an unspecified disaster happened which razed humanity to small remote fragments which are technologically comparable to the 18th/19th century. I wanted this world's nations to be composed of island nations without the existence of substancial landmasses. Then I realised that this can't happen IRL just by the melting of ice caps. Other explanations would be: Nuclear war rendered continents and specially the northern hemisphere inhabitable, so people settled in south Pacific islands. Space-mining operations caused massive meteor showers that brought lots of extra water from space. There have always been massive amounts of water trapped deep in Earth's mantle. Underground construction and the search for geothermal energy caused this water to be released. During the construction of the ecumenopolis, large portions of continent were dug out for resources and replaced by floating platforms. The disaster destroyed and sunk most of the city infrastructure, revealing the "swiss cheese crust" bellow. My story is supposed to start a while long after this disaster, thus civilization had rebuilt well ahead into something similar to modern IRL world. This is why I'm posting as just "After 1900" and not the future. I want you to pretend this story to be a post-cyberpunk version of the modern world which arised from a point of divergence in at the start of the 20th century. The caveats is the differences in available technology, resources and culture. I imagine the atmosphere to be like "20 minutes in the future", where people have lost knowledge of their history and mention the OTL as "the ancients" or "the forerunners". Multiple skips in the tech-tree are explainable by the fact that people are retrofitting and hacking together remnant artifacts of the futuristic (or not necessarily) OTL. In the midst of this train of thought and planning, I have concerns about plausibility. So here's list of linchpins I have issue with in list format: The forerunners used up all the fossil fuels, polymers, chemical fuels and advanced pharmaceutics don't exist. This affects chemical industry in general. Great efforts on genetic engineering and organic tech have been used to compensate the shortcomings of the last point. Nuclear reactors (specially RTGs) have been greatly miniaturized and are the main source of energy in the same format of today's electrical batteries. The last point enabled a large jump in the development of electronics, so all propulsion is based on electricity. The last point has enabled powerful and exotic computers and (although specialized) biopunk robotics. The development of General A.I. is feasible. There are little or none personal vehicles, thus no large roadways. Cities are small enough for walking or short boat crossings. People that need it, use the sparse public transportation and most vehicles (usually air-carry) are used for industry. Due deficiency in chemical propulsion tech, there is no space-faring and so no satellite based technology. Earth's orbit is known to be cluttered by Kessler syndrome and people don't bother attempting to contact existing satellites even if they still work. Due deficiency in chemical propulsion tech, knowledge of gunpowder has been lost. Criminal undergrounds have kludged together primitive firearms but nothing better than that. The military/police has returned to use bladed weapons, along with using robotics, yet meanwhile, there have been recent developments in exotic weapons which behave like modern firearms. The nature of these firearms is open to discussion. I'm think about discussing each issue by itself in other posts if there seems to be interest. Meanwhile this post will be used as introduction and to better structure these ideas along with your feedback. This might be unrelated to Alternate History, but you might like to know that I've also been working on a constructed writing system to go with this civilization. You can read about it here at Omniglot. It reflects the culture of the fictional society and so it might be worth considering.