Recently I played a game called FarSky. This isn't a review of FarSky, but instead about an idea it inspired.
Originally I wrote a long, meandering essay on the hows and whys of this idea, but instead I'll save you some time and just present it straight-up.
You live at the bottom of the ocean. You have a science fiction backpack - it's an electrolysis machine that can put out quite a lot of atmosphere, limited only by battery life. You can recharge batteries from any power source, or deploy a very long thermal wire to float high above, and sit still for a long time. Of course, as long as you have battery power, the backpack can keep you breathing, but the real use of the backpack is to fill up aquatic bases.
You can make an aquatic base out of anything. Shipping containers. A cave. A sunken ship. An abandoned research base. Sunken houses and vans, if they aren't too rotted. Subways. Any place that is vaguely airtight. Activate the backpack and pump air into it. If there's holes, the water level will stop dropping at that point and you'll need to patch them to put more air in. If there's multiple rooms, maybe you close the bulkhead and fill one room at a time, slowly conquering the base bit by bit.
The issue is that you're not really very useful. All you really do is fill things with air and patch small holes. You can't drag heavy things around, or mend a shattered window, or cobble together a bed, or grow plants. You need NPCs for that. And, of course, the NPCs need some place with air to stay.
Not all locations are created equal. Like most survival games, as you become better-equipped you'll want to move further afield, into areas with unique new resources. Unlike most such games, the bases aren't completely stable. While there is some advanced tech to help scrub a base's air supply, fundamentally they only have as much oxygen as you put in using your backpack. That means that you'll want to keep your NPCs consolidated to just one or two bases, or you'll spend all your time running around refilling your bases rather than exploring. So as you march onward, you'll need to abandon one base and move to another in a new location. Fortunately, as long as there's decent resting stops along the route, the NPCs will happily drag all the stuff in the old base to the new base.
The early part of the game will be mostly to get you used to the idea of moving around the ocean, finding small places to fill with air, acquiring one or two NPCs to help you with basic crafting. You tag resources you find in the open ocean, the NPCs will go fetch it and bring it home, allowing you to steadily get a grasp on how that sort of thing works.
But the gameplay of expanding into a base is the heart of the game. Reclaiming a base room by room, patching the cracks you can patch, sealing off the rooms with larger holes. Creating an airspace, moving your NPCs into it. Having them patch up the holes you can't handle, arrange the furniture. Going out to find things like shipping containers or vans, tagging them for recovery, and then attaching them to your base to steadily expand it. Making your base larger has purpose even if you don't plan to put any more stuff in that space: every additional airspace volume you add is more time the base can go without being refilled.
The NPCs have classes, tiers, and levels. For example, an NPC might be a retrieval specialist, able to quickly move large objects. Or they might be a craftsman, capable of building furniture. An electrician, capable of repairing or creating complex devices, including tools other classes might need. A plumber, capable of sealing up holes and creating infrastructure for things like toilets and showers. Many others.
Levels steadily increase as time passes. The higher someone's level, the faster they accomplish tasks.
Tiers are based around quality of life. If someone is living huddled in a shipping container, they aren't going to be capable of doing much work. But if they live in a vast base with all the amenities, they'll be able to create masterpieces rivaling anything that was created on land.
Hopefully the framework of the game is becoming clear. A cycle of expanding into bases, improving existing bases, scouting for resources, getting NPCs, and shuttling NPCs between bases.
I think this could be really compelling, but the flavor would need to be managed well. You'd want to create an environment where swimming around felt really compelling. You'd want the steady way you convert a base to feel heavy: you go from swimming in these rooms to walking in them, little by little. You'd want hanging around with NPCs to feel relaxing and interesting, so you feel like these are people you would enjoy being stuck at the bottom of the ocean with.
Hm. I'd love a game like that. Maybe I'll program something like that someday, because the basic concept is a powerful one that isn't really explored much.
There are games where you cobble together bases, but it's typically just attaching individual pieces and then they work. This is being inside the base and steadily filling it with air one room at a time.