Utterly Mad

Apocalyptic Future/Dark Future/Far Future

Cold war never ended, Russians won the space race. By mid 2020s, humanity moved out to the stars to escape threat of nukes and stuff. Earth nuked itself and has been lost to time as just a legend. Thousands of years later, "Russians" and "Americans" still carry out an intergalactic cold war, about to turn hot, even though neither actually remembers anything about the original communism vs. capitalism idological stuff, instead hating eachother because that's normal to do. Maybe some other threat now presents itself as well, threatening to fuck up the balance of power and kill everyone.

Humans colonized everywhere in the 2100s, finding the galaxy surprisingly empty. Bad thing A happened, and everyone lost contact for a LONG time. Some confederations formed, some planets forgot they aren't where humans came from, some people evolved into things not recognizably human anymore. People in some places have gotten over bad thing A, and are moving out into the stars again, finding the galaxy much less than empty now.

Humans tried to colonize the galaxy, but it turned out all the good spots were taken. Shouldn't have poked the alien bear, so some titan AE shit happened. Thousands of years of war, scrounging, and being on the run, humanity is on the verge of extinction, and every day another world collapses.

Humans bumped into benevolent aliens on their first extra-solar stop. Aliens shared everything with humanity, setting up thousands of years of prosperity. Jealous faction created a virus to kill aliens because they saw them as X (slowly killing human culture, too fascist, too communist, whatever), and it worked. Human Empire is slowly decaying and corrupting itself.

Humanity bumped into seemingly benevolent aliens after slowly moving out of the solar system over hundreds of years. Aliens found a clever way to slowly kill off humans (lowering birth rate, mysterious illness, whatever) while still seeming to be friends, people figured it out, galactic war insues.

Humans are the benevolent aliens. This is not quite as pleasant as it sounds, because aliens don't always really want benevolent overlords.

Apocalypse

China and Russia went very, very sour during the sino-soviet split. Nuclear war isn't fought Commies vs. Capitalists, but commie vs. commie over control of the red world. China wins and starts consolidating, but the west sees it's chance and nukes them. They get nuked back. Everyone nukes everyone. The war never actually ends, but people are just to dead to fight actual battles anymore.

As above, but the war does end, with communism coming out slightly on top. As part of peace deal, U.S. (only western nation not a glowing wreck, sorry "stuck in the middle" europe) is broken down into small republics, kinda like Russia right after the end of the cold war.

Algae bloom renders most water undrinkable. No major nuclear war occures, but small scale wars are fought over remaining water rights between neighbours. Someone finds a way to clean water after most people are dead, humanity struggles to rebuild and not nuke eachother now that the water thing is out of the way.

Nukes are never developed. Without the threat of global annihlation, the West and East get into it about 10 years after the end of WWII. Gas, germs, and bullets kill millions. War never ends, but yet again, most people are too dead to care or keep fighting. Europe makes a better setting here, because it seems realistic to me that that's really where this would be taking place, but a depopulated, slowly collapsing US could be interesting too.

Some third world party develops a super weapon (super nuke, biowarefare, whatever) and tries to end the cold war. No one takes them seriously. Third world tries to demonstrate just how serious they are, something goes wrong, everyone dies.

Turns out nukes do actually ignite the atmosphere and boil the oceans when you make them big enough. After years and years of "who's is bigger tests", this goes awry and everyone dies without an actual war taking place.

Nutty group starts a plague with specific, if absurd goals. Ends up killing everyone because plague weapons are a pretty bad idea.

Nothing identifiably bad really happens, but people go the way of the panda and just stop living and reproducing. The world crumbles slowly without anyone noticing untill it's too late.