Game changer: the ability to tax trade routes

This mechanism shifts the dynamics from straight up plunder to one of commerce as well.

Imagine an 8 player free-for-all scenario. You have an adjacent civilization that wants to trade with his ally across from you. Said adjacent civilization has the option of sending his trade carts through a long and treacherous path or he could go through your civilization and every time he passes your gates you charge tax.

This will create a Silk Road type scenario.