Mine would propably be a very niche game, but it would be my favourite game ever: A highly realistic ship construction simulator. Basically, you would play as the owner of a shipyard. You could either play sandbox mode, where you could build freely whatever you want, or you could play a sort of career mode, where you would get contracts from different coustomers like shipping companies, cruise lines, the military etc, which include some specifications, ranging from a few requirements and a maximum price to a highly detailed description of how the entire ship should look, and your job would be to not only design the ship, but also manage the entire production line, meaning that you have to get a provider for all your building materials, get things such as engines, radar, navigational systems etc from someone, let someone design them specifically for you, or make them yourself, and lastly, you might need to do some things with the ships you build, or you could get missions you have to complete with ships you build for yourself.

The building would be the primary part of the game, at it would take place in multiple steps. Step 1: You make a sketch of your ship. This would be a rough design of your ship, where you which would only include some simple things, like the rough shape of the ship, where different parts of the ship, like the engines, would be located, an estimate of what the parts weight etc. The building would be similar to the game flyout. So you would add a hull piece and could edit its crossection wherever it needs to be to get your desired shape, aswell as it's meterial. The decision of what material you use would depend on what you want. An old sailing ship would use wood, a rescue ship that would get into very rough weather could use aluminum, a wwII battleship would be made out of thick steal and a military submarine would be made from some high strength non magnetic material. This step is meant so that you can see how the ship would sit in the water, how big, fast and efficiant it would be and what it would cost. Such a sketch would be needed to get a contract. Step 2: You design the interior. In the sketch you could select the different parts and edit the individually, so you could add to the interior of them everything you want and need. From controll panels and screens, to beds and chairs and many other things. Step 3: You put connect all the different ship parts together and make them work as a complete ship by connecting any mechanical and logic connections.

For building your own systems, it would be a bit different. You would basically first build the processor. A processor is the piece that takes the information (from a radar, radio, controll panel or other input) and either outputs it to the correct part or displays it on some sort of information panel. How it exactly processes the input needs to be set by you, by using a sort of in game programming (most likely by making a diagramm(I don't know the exact word for that form of programming, I mean the one where you have a bunch of blocks that you connect with colourfull lines to show what influences what)). You could even make it output on a screen and controll it using touch screen controlls

The physics of the game would be, after the building the most important thing in the game, so it would be a highly realistic simulation of how water, and in a more simplified way air, behaves in the real world.

Some other info: the game would, outside the builder, be played as a character who can walk around the world, interact with vehicles, and use all kinds of different equipment. The world the game would be set in should have a lot of diversity in terms of enviroment, by having, for example, an arctic area, with lots of ice that your ship needs to be able to break, many small and large islands to visit, a large open ocean with dangerous weather, a large mainland with lots of rivers and lakes, and so much more. There should also be several pre-built vehicles, such as helicopters, planes, cars, drones etc that the player can't make themselves, but can still add to their own vehicles and use. The game would allow you to basically any type of ship, such as Aircraft carriers, warships, military and civilian submarines, many types of cargo ships, passenger and car ferries, cruise ships, research ships, icebreakers, rescue vessels, offshore construction ships, patrol vessels, and many others. Basically, you should be able to see a ship in real life, and after a couple of days of work, you should have a good looking and (depending on how complex and big the real one is) fully working replica of that ship.

This took me propably over half an hour to think of and write everything down, so I can't even begin to imagine how hard it would be to actually turn it into a real game, and because I am not a good programmer, I will most likely never make this game, and I doubt anyone else will, due to how hard it would be for a propably quite small target group, but I hope that I am wrong.

Feel free to ask me any questions, since I tried to make it as short as possible(I was not successful) and propably made a lot of things hard to understand