Perhaps I am asking too much from this, but this is really something I would want. I tend to want this in almost every open world game but not has yet to really achieve it. (to my knowledge). Also this could kill minecraft for plenty of people so I would like this to be optional in the menu.
1. I want a dynamic world filled with living NPCS that build, harvest, interact, explore etc. Now most would tell me to get some mods. However I want NPCs out there who may get together, grow in population and literally build up their own shelters, farms which will eventually grow to a town as they gather more. And if you live in this world for a LONG period of time it may develop into a city with large buildings etc. This feature will give true life to minecraft.
2. In addition I would also want fully interactive furniture. A bed to sleep on, chairs to sit on and library shelves that allow you to write books for any reason and store them in. Enabling such an RP element would make this game a social network as well as a building/exploring game.
3. With point 1 and 2 merged you will have quite a Town! Then, to a even more complex feature I would die to see these towns literally build up communities such as husband/wife, develop a law so you cant steal from anywhere without getting the villagers on to you and maybe even have a NPC trading post. They will have supplies on offer to purchase if you trade them something they find of value (maybe they lack steel etc). This feature will give towns and NPC purpose. But also, instead of growing a stupidly massive amount of items your not going to use you can feel like a true miner delivering minerals to a growing town and receive rewards for doing so. Could you imagine several towns? What if there could be war? A player built (in multiplayer) town who act as bandits who camp near developing towns to raid all their supplies once they have the man-power?? But I know I'm asking too much and this isn't the game most are addicted to. But if only... if only...
Java probably couldn't handle that... But minecraft's open world is probably the only game which can allow you to create a game like that. Every other game has severe limitations.