Oh also in response to your first question Mica: Agriculture makes no sense because nothing grows while you're offline. it's very hard to get a farm going unless you leave your machine running all day and night.
OPTIONAL NPC's, Something that can be turned on or off when a game/server is set up, In my opinion, would be very good for small servers with few people or for those who are playing alone. Have NPC's perform specific jobs and sell and trade their good to the player or players in the world, perhaps something where the players would have to build houses to "attract" NPC's to the area and then the NPC's could build their own "workshops" onto the house.
It would be interesting to see if aircraft could eventually be added into the game. It would be cool to have to build a landing strip for said vehicles, and use them to transport goods, equipment and people. Plus it would also add a nice alternative on larger worlds or reaching remote area's