Since everything is user-generated, you create the world and the quests.  The basic structure of the game world is like this:

• There is an overhead map view of the world.  The map has different kinds of spaces on it.  Grass, water, buildings, etc.
• When you click on a space, the view zooms in to a 2D side view of that area.
• If the area has a building, you can enter it.  Inside the building is a room.  A room can have doors to other rooms.
• A room can also have objects in it.  An object can be anything: a chair, magical orb, wizard, etc...
• The player can carry some objects, these are called items
• The player can talk to other players in the same space and exchange items

When a player interacts with an object, one or more of these things can happen:

• Display text/dialog.
• Give the player an item.
• Disappear (die)
• Turn into a different object.
• Do the above things based on the player having interacted with some other object.

So given the ability to create buildings, objects, define what they do, and chain them all together to build a quest, what would you create?