I feel like I've done enough commenting on current MMORPG's lately, let's talk design.
Minecraft (And Terraria, for those who keep up with those games) have created a system in which if gives you a random world. Fairly simple to do, given that the world is made of blocks and placement of resources is randomized. Each world can be hosted and joined by other players, such as Minecraft allowing you to create a server.
Now, creating a central hub to turn this into an MMO is the easy part: Simply make a city decked out with the usual merchants and auction houses and that would fulfill that need. To accomadate the potentially limitless areas one could explore, you could take a page from D&D lore and create the City of Doors, in which there are portals to an infinite variety of lands. This would not only create the in-character reason for exploration, but also remove the restriction of keeping it fantasy based in the event someone wants to build a star destroyer.
A larger problem exists in one of the main problems of Minecraft servers: As in, griefers destroying what you've created. Here we are led to an interesting issue. If we allow players to password protect their lands (either by keeping people out entirely or simply protecting them from altering any part of the land) we give them a means to protect themselves, but not necessarily to create meaningful content for others beyond sightseeing or helping you build something (which they can do in their own little realm).
Which then brings us to what must then contain the bulk of the content: Enemies that assault you, at whatever time during the day. Randomized boss fights, the possibility of having your creations destroyed (mind you, this feature could be turned off for those who only want to build).
How about this then: Build-able items that attract monsters to them. This allows players to create their own dungeons or monster hunting areas, but also restrict them to certain areas and the monsters would still be randomized. Treasure could be found while mining underground, or from randomly dropped chests from monsters. Or built, of course.
I think this might work out. Thoughts?