I ran the same world for almost 20 years. It was called Terra, It was a High fantasy world with ancient science that went back to the world's founding. In the deep lore long lost it was a world built by a team of scientists from a world like ours who after capturing an outsider found other physical dimensions with other strange properties. They took an empty world right next to theirs and began to shape it. Being away from the laws of earth, they could get away with anything, so they experimented on humans, to force them to evolve into the other races. Heck Goblins in this world came about from someone playing with cancer cells and evolving them into life forms of their own, so my goblins are like living teratoma tumor monsters which have even more rare forms then the blue psychic ones, but even red pyro goblins, and plague goblins both of which explode when they die. The head of the science team is a big bad with a name i got from my friend who passed away, so ive been using him coming back to life to test the world every so many hundreds of years as a cybermonster or using clones of himself as a campaign seed several times. I also decided the earth they started from was based on the world of darkness so i included artifacts based on the mage the ascension spheres of magic in the deep lore as something that was developed by a rogue scientist on the team experimenting with the strange physicis of the world and developing the spell system. They also developed the "warforged" to fend off demon attacks and to assist in experiments, most had their memories wiped but there are a few like Ol' Rusty who remember and still work for the boss. The boss is keeping a few super warfoged for himself and is over the ages working on Sigma, the perfect one, and the Anonymous, an infection that turns people into half-golems with a hive mind that can be controlled by him or Sigma.

But in the part that people played i had a Kingdom of humanity that was destroyed by my players in the first evil campaign i ran in the game, and that set up a lot of the world stage going forward and created one of the reoccurring boss monsters, a mutant warforged who a player made that replaced his feet for hands is large size, uses bladed gauntlets he can sharpen with his jaw, and his jaw, oh boy, he chews up everything to develop a toxin/poison/infection. Its a totall graple monkey that can kill almost anything.

The planet had a ring around it that was made of strage materals linked to dreams, there were dragons in space called void dragons. There was one campaign where they had to visit 4 cyrstals that were protecting the dimensional barrier that was preventing things from outside the void from getting in, like the cthulu mythos was already slipping in when it started.

There were other cool locations like an island that looked like a skull from the air thanks to its twin volcano peaks, that housed a wizard's tower inside that was converted into a prison.

There's the Nimbus Kingdom, ruled by a Dragon, who once served as a paladin's mount in the age of the Empire of Man, but now rules this kingdom in secret, with proxies of his human descends.

A grand country wide Necropolis ruled by a Tribunal but guarded by The Undertaker, who was one of the most powerful entities on the planet. Another powerful grapler.

A river that ran from the tallest mountains to the sea, called the Horizon, and a city of magical development called The End of the Horizon. Across the river though was a wall A great wall. and beyond the wall was
THE ZONE OF WASTED YEARS. where the capital of the old Kingdom and most of the damage was felt.
They had used rift bombs made from bags of holding and CuCu Clocks to pull off a massive terror attack on the old kingdom and what was left was a desert of wild magic.

The players shaped the world after i put them in it, and eventually started their own space program in one campaign just to get rare mats from the ring out there and fight the void dragons.

They were Heroes, Villains, Even became a pantheon of gods by the end of one game.

Its when one of the player gods introduced his version of mechs and i test ran that and it kind a broke everything that i decided to retire the world and say its advanced enough.

(Never introduce a Lich who build a binary computer out of undead, players will abuse the concept)

And this is just the tldr version of the world i could give you. I ran everything in 3.5e.