Ok you convinced me :)
I have to work on it now. One decision point is about general principle : consider the market with the most population (main market) then update population demand for all markets, or consider the population in the region (i.e. sum of all markets population), or each market has its own demand according to the population (small markets have less needs than big markets).
I was thinking about an other way to interest player to start playing early rather than wait until all resources are shown: buildings and traders leveling. The more an element do its job the more experience he gains. With experience he level up and improves its characteristics (auto or selected by player). For example, the more a trader travels the faster he goes, the more a workshop produces the less time it takes to produce...