Caravans

[hashkage]

It would be cool that if you were to send resources to another player, you had to send a crarvan which could be attacked on the way. Though there should aswell exist "safe regions" where the caravans cant be attacked, though it usually means taking a detour.
Faster trade, higher risks.

[Darathor]

While you can trade resources, I don't know whether you have to send them a caravan, just trade it through a menu, or you have to go a market in an npc city. You do, however, have to send a caravan to an npc to trade with anyone else for resources, and those can be attacked.

[hashkage]

Cool. Thats an even better idea than mine .

[szebus]

This one is good.