Something that might make for things worth trading for:
If crops and animals were not all found in one place, and particularly if crops were only able to grow in particular climates (at least by y-coordinate if not more specific than that). With different plant species available to grow in different major towns, the towns would find themselves wanting to trade with each other on a regular basis for the plants they didn't have, if these other plants were at all useful or necessary.
For instance, spices that boost happiness (when it does something useful - right now for me it just trends downward, but I've never had tea), various kinds of fruit trees and so forth, a northern land inhabited by killer walruses instead of foxes... :3= (Okay, the last one is a bit silly)
However, there's a problem with this, and with trading over large distances in general, I think: Being sent back to the hearth-fire because of a crash, disconnection, or other such problem. IMHO it'd be better if, when you logged in, you had a choice between going back to your hearth fire or re-appearing where you logged out.