I think a good system might be where every good has a price in each CoT, and provinces choose which good to produce based on how much "profit" they can make (IE units of production*price, with different goods having different fertilities based on province terrain and climate). Alternatively, allow players to change production in a province manually, in a limited way.
Different possible goods, province terrain, climate... Not to forget some meaningful system of how many units of a certain good can be produced (i.e. taking into account population and transportation), regional markets and money made by trading between COTs with different prices for a certain good... It would be very nice to have that, with influence of availability of goods having other than financial effect on faction...
TBH, that's more Vicky territory, and it is probably wise to keep it that way. Apparently, the Vicky II devs get a fit whenever regional markets are even mentioned.
To be a bit heretical: wouldn't it be better to get rid of trade goods tied to provinces? ATM, there are some arbitrary boni for having access to certain trade goods, but that's pretty minor. Some events are influenced by trade goods, so what. The biggest differences is the production value and the trade value, which feed money into investment (or minting).
Instead, incorporate some of it into the base tax (i.e.roduction) and cover trade by collecting tolls. Which of course, would demand a different trade system, but that we need anyway.