I recommend procedural content generation. As in
- make random tables to fill your world
- make rules to make it behave and interact with itself
- create a slice of "world"
- explore and interact with it
- keep a blog so people can read about it and play it :)
It's what a roguelike does. I'd start with generating some cities and some goods to be traded. Then you can generate the rest of the map as you explore it and find new trade routes, bring artefacts from long lost tombs back to civilization and act as messenger between distant civilizations. Sounds fun to me :)