An MMO where you can actually create BRAND new items, metal alloys, armor and the like. Possibly even advance into the industrial age and into the sci-fi stuff if we're going that far.
And of course, building, economy, wars and the like.
Magic is optional, but would spice stuff up a fair bit.
Imagine the following.
The game is fairly early on, and everyone uses "conventional" iron swords (although with fancy blade design) and some blank armor.
THEN comes the guy who, after working in a small mining town, strikes the earth tremendous amounts of a rare mineral with (as-yet) unknown properties.
He then sells it for a fair (but previously unset) price to a local blacksmith, who then forges it into a supposedly-typical sword that even later on is bought by a warrior of some sort.
But when he decided to go hunting for wolves (or something, to feed the aforementioned guys, who I will assume all live in the same village) he notices that the blade lit itself on fire when he pulled it out of the scabbard, which for some reason was made ouf of a material that makes the metal the sword was made out of to start to burn (for sake of simplicity let's say it's copper, I dunno why, though). He decides to screw the wolves and runs back to the blacksmith, all confusion, panic and eureka aside.
He tells the blacksmith (who would be another player, that is) to smelt the sword back into metal bars/whatever and reforge them into arrow tips instead and to also craft a bow out of copper.
Then he gets the arrow tips to the local bowyer that finishes the bow and the arrows, kinda confused about what they were made out of, but he didn't really pay much attention.
Once they're done, the warrior takes the new weapon for a practice run, and is successful - the arrows light themselves on fire when fired from the copper bow as they touch the copper, resulting in nasty and deadly weapon while everyone else is still running iron and wood.
Then they start to mine for of the mineral, buying copper from other villages (assuming that there's no copper in their own mine) and creating the deadly weaponry that they later use to form a small, but efficient military out of, with custom banners and dyed leather armor, and the warrior eventually becomes its...Maybe not yet a king, but a leader, effectively forming a country. (or at least a city-state, since I'm certain that something as unusual as flaming arrows of magical metal would attract a fair society)
So essentially a sudden discovery led to creation of a brand new weapon type and a new force arising.
That is of course, just a mere tip of an iceberg.
Oh, and there are NO NPCs. Everyone is a player. Everyone can and will be killed. Then you can even get to the space era with spaceships, new planets (and the space-era players discovering a planet where another group of earlier-era players still live, which would make them aliens in a way).
I don't know or care how hard it would be to make it work (VERY HARD!), but I'm sure it would be worth it a thousand times over.
There was a similar idea many pages back, but I was too lazy to look for it, or just didn't know what to actually search for.