I don't know what would get me to come back to the game (besides my friends nagging me.) It would definitely have to include new elements and challenges. It would also have to be an Express server. I wouldn't play a 12 week game again.
I like the idea of a world map as somebody suggested. I also kind of like the idea of forced mixing of players as another person suggested. Servers die when the same teams play together over and over and nobody bothers to challenge them. (instead, everybody flocks to their city. )
What if it was a world map and the were different daily or weekly challenges that needed to be met surrounding import or export of goods that are only found in certain parts of the world.