I think trade has some issues. Trade routes appear to be chosen at random, and they don’t have direct social or political consequences they should, such as making trade networks or silk roads or allowing units to travel on them.
Total War Atilla has a nice proto-idea, where roads are made not by workers but automatically between settled settlements. Having trade routes appear as a network on the world map and them interacting to affect roads, be pillaged, or cause socioeconomic effects would be interesting, and would allow modders to improve on that.