Even if it's simply your reason why you think this game is impossible.
not impossible. it'll just take a REALLY long time.
imagine the better parts of Star Citizen+Civilization+Grand Theft Auto+Halo
that means roughly 1500 years worth of dev time, most of which is done by veteran experience in their specific fields. People with potentially several decades of experience, studios with game engines, assets and prior work to use from many many earlier games, across a multitude of disciplines.
multiply the team size by the years of development for each of those games and add them up. that's how long it will take in man-years, once you have the skills and tools, and a way to support yourself while you build it.
if its 1500 man-years, and you can get together a team of 10, you can do it in 150 years. a team of 50, and you're down to 30 years, so its not impossible - just not practical.
The sad fact is that big games are a lot of work to build. the original version of Skyrim (not SE) is about 500 man-years. (team of 100, 5 years dev time).
its on the order of building a destroyer or cruiser - but not as much work as building a battleship.
and in terms of parts, if you consider each character of code a part, its probably as complex as the space shuttle, which is one of the most complex machines ever made at over 2 million parts.
so, all that being said, how do you proceed to do what you can?
step one, you'll need an engine. something that's suitable for both flight sims and shooters. so start downloading and checking them out. think about what you want your game to do (seamless worlds etc), and see what engines can do it. then start playing around with the engine prototyping things. just learn as you go. starting from scratch, it will take a couple of years to get enough skills in code, paint, modeling, and animation to begin to really do stuff. and don't forget foley and music!
one thing you can do is design your game to require less man-hours.
procedural level generation means you don't need level designers. and all those man-hours of hand editing the game world disappear.
another thing you can do is massive re-use of assets. construct objects from shared meshes, textures, materials / shaders and animations where possible. Caveman 3.0 implements a Paleo-world simulation covering an entire continent with < 500 meshes and < 500 textures total for the whole game.
purchasing assets is another great work / time saver. $100 for a decent skinned mesh beats months of learning to do it yourself. and the time required to learn character rigging and a animation is nothing compared to learning character modeling. many tools come with assets you can use in your games. there are also special tools you can use to create assets faster.
Procedural generation of quests and missions means you don't need writers for quests / missions, and you don't need quest / mission editors.
You may not ever approach the combo of those you listed games plus more, but you can go a long way towards your goal. just take it one feature at at time, starting with whatever is the most important feature at that point in the project.
make a list of features you want, prioritize it from most to least important. add design notes to features so you know how to implement them. this is your todo list. implement a feature, then test, debug, refactor if needed, then move it from the todo list to the done list. then on to the next thing on the todo lsit. project scope is so large you'll never "go gold", but after a few years, you might have some pretty cool beta's you can post.
FYI: Caveman might be a good example of what a solo dev can do. its a seamless randomly generated persistent open world survival game with a stone age setting and an emphasis on realism. It combines FPSrpg gameplay as seen in skyrim with person sim gameplay as seen in The SIMs. you can interact with just about everything in the game world (a la the sims). the world map is 500x500 miles in size. there are 60,000 caves (settlements or hostile encounter areas), 5000 rock shelters (settlements or hostile encounter areas), 18000 huts (settlements), 20000 caverns (dungeons), and 40000 special encounter areas (valley of the tigers, etc). about half a dozen core stats, about a dozen variable stats, about 50 skills, 1-2 dozen types of resources, 300+ types of objects, all off which can be crafted and repaired - including about 65 types of weapons. 50+ differnt species of wild animals with unique models. it uses a custom game engine based off of DX9. its been about 3 man-years so far. and i have 35 years experience building PC games. from scratch it would probably take one about 3-4 years to learn all the coding, artwork, and audio skills required. and even then the game is not exactly AAA quality - especially the graphics. I've just begun working on the final graphics, now that the rest of the game is pretty much done.