An idea for an MMO game.
I had been thinking about MMO adventures (Runescape, WOW, ESO...), and while I did not play any of them, I did think of one strange thing. Quests can be done by multiple players sequentialy. (Player 1 does it, gets reward, player 2 does it gets reward.) This is strange when a quest is killing a specific creature or anything else that can only be done once.
But I see why this is done. If most quests could only be done by one person, the devs would have to either keep adding quests, or later players would not have any quests to do.
So the idea is, replacing all NPCs with PCs. A player could either play the game as an adventure game, or as a idle/clicker game. A character could only master a certain amount of total skill, so a master smith would be unlikely to also be a master swordfighter. And if he was, he would have no other skill, so he would have no idea how to shoot a bow or to tan leather.
Most items would have to be made by PCs. The sword that an adventurer buys in town, he buys from a smith, who had to buy iron ore from a merchant. This merchant bought the ore from a mine in the mountains, mined by miners.
There would be creatures, randomly spawning in wilderness, some of which would attack PCs, so traveling merchants would need protection.
Some of these creatures could even attack cities.
If PC loses all HP to hostile creature, the character is dead and the player needs to make a new character. (But all items stay on the corpse)
If PC loses all HP to provoked creature or accident, they are merely knocked unconscious and will recover after a while. (But can be robbed with impunity while out.)
If PC loses all HP to another PC, the victor has a choice to merely knock them out or kill them.
The idea is, that random creatures and players themselves, rather than predefined quests, would provide content. I would expect some players to be playing as criminals, robbing merchants and the like (but preferably not killing them, as that would just reduce the profitability of crime in the area.), which would give merchants additional incentive to hire guards.
And some trolls would decide to troll people with terrible deeds, becoming villains. But then someone would probably hire heroes (or at least antiheroes) to stop them.
There would be a magic system, which would certainly have to allow necromancy, possibly also ressurection. (If you can get a corpse to master healer within certain amount of time, he can ressurect the PC. But such healer would probably be just a burden on an adventure.)
This whole thing should probably be combined with a physics system allowing anything to be damaged. Natural terrain would regenerate over time, new trees would grow, but artificial structures would have to be repaired or rebuilt. Heated fights in cities could rack up some serious property damage.