moonburn´s idea was actually quite cool, but if it´s not possible, it´s not. Now, how about this:
Human player needs merchants in particular neighbouring regions, which he doesn´t control, but has a trade rights with, to get access to several ressources in an apropriate quantity to, lets say, provide his troops with superb armour ( ability to build and maintain the forge ), or horses ( to build and maintain stables/recruitment option for special units, wich are on the unit list of that faction, but are only available in those neighbouring provinces if the a player makes efforts to control the trade ).
If a script could damage the buildings, or cansel recruitment ability for those special units, every time a player loses the "monopolistic" control of the trade route ( just like some Roman barracks ), it should be possible to tie some special, happieness buildings, for example, to it too, so that once you loose control, you lose the cool features.
I know, it´s possible to create a dependence on trading goods to enable the settlement to "produce" the " follow products". Metro Naval Mod had this kind of features, FoE had them too, iirc.
So, the basic idea is: if you wan´t well equiped troops, some special troops from neighbouring region, or the benefits of grain imports en mass to boost up your population growth, or plentifull slaves for yur Latifundias, you should make some efforts to outmaneuver the competition :)

Hope i didn´t repeat some allready obsolete ideas, which has been denied.
AI, if it can´t "learn" it, would gain all the features anyway.