I have a game concept around here somewhere... it's basically a living fantasy world.
There are no pre-defined quests, no stories, nothing. Instead the entire world is alive and breathing. NPCs go about their life, they have preferences, they have money and an income, they have jobs, they have hobbies.
You earn money and gear by helping people, there are always tasks to be done.
You are able to manipulate that environment. For example... you have a city guard who's into redheads. If you do not interfer he will eventually end up marrying a cobbler's daughter. Or, if you get his partner killed, he'll comfort the redheaded widow... and they end up together.
You could use a kid as messenger, paying him well to run errands for you. Or if you don't, then the kid might starve. Or go to school and learn a trade.
At first you're barely more than a vagrant, doing odd jobs for a little coin. Splitting wood, gathering herbs... and as people trust you more and you become more capable, they'll ask you to hunt the wolf which was killing sheep. Maybe some traveler got robbed by bandits and needs your help getting their stuff back. More people will rely on you as your reputation and skill increase, eventually you'll be killing dragons for a king.
The game doesn't even need a main quest or story, it basically writes itself. If the world is large enough, then a few seeds will grow into terrifying opponents over time. Maybe the disgruntled wizard apprentice stumbles over a tome of dark magic and by the time you are a hero, he'll be a dark wizard sitting in his tower, sending out monstrous minions to terrorize the lands. Perhaps the apprentice messes up a ritual and summons a demon lord he can't control, who kills his summoner and sets himself up as a king.
Of course, nothing in the game says you need to be a hero. You can just as well become an evil overlord yourself. Or work for one.
The actual concept is a bit more complex and it is unlikely current technology is up to the task... not to mention that it would be a lot of work to code it.