I've heard a lot of people say they're bored with Minecraft now.
How could Minecraft be boring, when there are so many things that can be done?
Minecraft is the ultimate sandbox game, and this will only make it better.
NPC villages are cool, but there is a strict limit to what they feature, and jungle/desert temples are visited once and then disregarded. A custom terrain generator would allow the user to choose which time period and place in the real world, or, a fictional place, such as Narnia. Not only would there be specific structures for the time period and place chosen, (example: Europe, Medieval) there would also be certain items that would only be craftable in those worlds. (example: Europe, Medieval would have a Flail, or a javelin, or a king's crown) Servers that have multiple worlds could use this so players could play in different time periods and switch between them.
Different world generation combinations that could be used:
and, of course, default Minecraft would be an option.
Provides diversity to the structures that randomly spawn in minecraft
New items specifically designed for the time period and place (example: flail in medieval europe)
Servers can have multiple different worlds and each can be a different time period
Possible changing of the default texture pack upon entry of a themed world (example: medieval themed textures in medieval europe, or new age style textures in industrial age)
Spawning of villagers in/around structures that trade the special items specific to each time period and place
Villager's appearance matches the time period and place
Possibility of themed worlds such as Narnia or Tamriel (Skyrim)