Alright so this came to me in a dream and the first thing I don’t want to hear is that this is just Fallout and Subnautica combined .
Let me set the scene, humanity has long been forced from the surface of the Earth by rising tides and war. Small groups of people managed to avoid the violence by escaping to the seas in underwater ships that eventually became permanent habitats at the bottom of the sea. There the remnants of humanity survived untouched from the changes the world was undergoing.
The main character is the first to set foot/swim in the areas outside the habitat since Armageddon(might rename this into something else like Sea Day or something..). Anyway… In exploring the outside world, the character discovers that much of what was taught inside the habitat about the past Earth has changed dramatically. The wildlife has become deformed by the radiation caused by the wars. Sharks have lost their skin and now are skeletal in features with large heads and slim bodies. New creatures that look avian roam the bottom of the seas for anything alive to eat, also skeletal. The ocean is much darker than the logs in the habitat recorded on the initial dives so many years ago.
The goal for the character is to lead/build humanity’s way back to the surface by collecting resources and clearing out hostile fauna from areas that would be useful.
Some interesting gameplay features would be upgradable explorer tools and weapons. Expansive civilization-the more discovered, the more the habitat can expand.