Thinking about the current trade system, and I think there is a simple idea to expand on the current system. It doesn't really need a major over-hall, but an expansion would offer some exciting management options.
Add an additional skill/technology to the game centred around a Scouting system.
Available from the beginning of the game, you would be able to research Tracking.
Tacking would allow a bonus to Hunting yields, in the form of faster movement and greater resource return.
Tracking would lead to Scouting technology.
Scouting would allow the creation of a single scouting unit, that you could send into the map to uncover fogged out areas (Fog of War style map should be implemented in the game irrespective of this new tech tree. This would add an additional element of exploration to the base game and realism) The scout unit could uncover trade routes to other civilisations either on the map as AI Villages or off map.
Scouting would lead to trading technology and inclusion of a market building. AI traders would now be received here, and you could create a second unit in the form of a trader that you could send out to the routes that you have uncovered with your scout unit.
The trading unit would enable you to nominate excess materials or finished goods that your civilisation produces, and barter them using the trading unit. You could nominate the minimum and maximum number of items to trade as well as items to barter for in return.
There would be a maximum of 1 scouting and 1 trading unit at this stage, and then a third unit from Iron Age, to allow you to expand on the trading network that you now have created. Both scouting and trading units would be open to the games natural elements and AI units allowing them to be killed, robbed, injured etc. Additional technologies could provide some insulation for these units. Scouts and Trader scan learn additional survival skills, fighting skills etc.
So in summary;
- Tech tree changes in the form of Tracking ---> Scouting ---> Trading
- Unit development in the form of Scout ---> Trader ---> Maybe even on to envoy or diplomat for later game play.
- Additional building ---> Market ---> ???
- Addition of FOG OF WAR to base game for realism.
- Addition of either ON MAP civilisations controlled by AI or map entry points to OFF MAP civilisations for trade route purposes.